rustc_codegen_llvm: Simplify update_target_reliable_float_cfg

This commit simplifies floating type handling through
`update_target_reliable_float_cfg` based on several facts:

1.  Major changes in behavior normally occurs only
    on the major LLVM upgrade.
2.  The first release of LLVM 20.x.x is 20.1.1.

Due to the first fact, we can normally ignore minor and patch releases
of LLVM and we can remove obscure variables like `lt_xx_x_x`.

The second fact is missed when the minimum LLVM version is raised to
LLVM 20 and all "fixed in LLVM 20" cases can be safely removed.

Author: a4lg

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -334,26 +334,18 @@ fn update_target_reliable_float_cfg(sess: &Session, cfg: &mut TargetConfig) {
     let target_env = sess.target.options.env.as_ref();
     let target_abi = sess.target.options.abi.as_ref();
     let target_pointer_width = sess.target.pointer_width;
-    let version = get_version();
-    let lt_20_1_1 = version < (20, 1, 1);
-    let lt_21_0_0 = version < (21, 0, 0);
+    let (major, _, _) = get_version();
 
     cfg.has_reliable_f16 = match (target_arch, target_os) {
-        // LLVM crash without neon <https://github.com/llvm/llvm-project/issues/129394> (fixed in llvm20)
-        ("aarch64", _)
-            if !cfg.target_features.iter().any(|f| f.as_str() == "neon") && lt_20_1_1 =>
-        {
-            false
-        }
         // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
         ("arm64ec", _) => false,
         // Selection failure <https://github.com/llvm/llvm-project/issues/50374> (fixed in llvm21)
-        ("s390x", _) if lt_21_0_0 => false,
+        ("s390x", _) if major < 21 => false,
         // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
         ("x86_64", "windows") if target_env == "gnu" && target_abi != "llvm" => false,
         // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
         ("csky", _) => false,
-        ("hexagon", _) if lt_21_0_0 => false, // (fixed in llvm21)
+        ("hexagon", _) if major < 21 => false, // (fixed in llvm21)
         ("powerpc" | "powerpc64", _) => false,
         ("sparc" | "sparc64", _) => false,
         ("wasm32" | "wasm64", _) => false,
@@ -366,8 +358,6 @@ fn update_target_reliable_float_cfg(sess: &Session, cfg: &mut TargetConfig) {
     cfg.has_reliable_f128 = match (target_arch, target_os) {
         // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
         ("arm64ec", _) => false,
-        // Selection bug <https://github.com/llvm/llvm-project/issues/96432> (fixed in llvm20)
-        ("mips64" | "mips64r6", _) if lt_20_1_1 => false,
         // Selection bug <https://github.com/llvm/llvm-project/issues/95471>. This issue is closed
         // but basic math still does not work.
         ("nvptx64", _) => false,
@@ -380,7 +370,7 @@ fn update_target_reliable_float_cfg(sess: &Session, cfg: &mut TargetConfig) {
         ("sparc", _) => false,
         // Stack alignment bug <https://github.com/llvm/llvm-project/issues/77401>. NB: tests may
         // not fail if our compiler-builtins is linked. (fixed in llvm21)
-        ("x86", _) if lt_21_0_0 => false,
+        ("x86", _) if major < 21 => false,
         // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
         ("x86_64", "windows") if target_env == "gnu" && target_abi != "llvm" => false,
         // There are no known problems on other platforms, so the only requirement is that symbols