const folding: fix libm not compiling

Author: Firestar99

crates/rustc_codegen_spirv/src/builder/builder_methods.rs

@@ -1492,6 +1492,8 @@ impl<'a, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'tcx> {
     simple_op! {
         add,
         int: i_add,
+        // TODO why does libm fail to compile if `add` doesn't support float? Why is it not using `fadd`?
+        float: f_add,
         fold_const {
             int(a, b) => a.wrapping_add(b);
         }
@@ -1503,6 +1505,8 @@ impl<'a, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'tcx> {
     simple_op! {
         sub,
         int: i_sub,
+        // TODO why does libm fail to compile if `add` doesn't support float? Why is it not using `fadd`?
+        float: f_sub,
         fold_const {
             int(a, b) => a.wrapping_sub(b);
         }