cranelift: fix register for srem.i8 on x86_64 (#5540)

* Change register written to in specific srem case. Add regression test as filetest case. Fixes #5470

* Add another test case, newline

* Update comment

Author: avanhatt

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -543,10 +543,10 @@ pub(crate) fn emit(
 
                 // Here, divisor == -1.
                 if !kind.is_div() {
-                    // x % -1 = 0; put the result into the destination, $rdx.
+                    // x % -1 = 0; put the result into the destination, $rax.
                     let done_label = sink.get_label();
 
-                    let inst = Inst::imm(OperandSize::Size64, 0, Writable::from_reg(regs::rdx()));
+                    let inst = Inst::imm(OperandSize::Size64, 0, Writable::from_reg(regs::rax()));
                     inst.emit(&[], sink, info, state);
 
                     let inst = Inst::jmp_known(done_label);

cranelift/filetests/filetests/runtests/srem.clif

@@ -142,3 +142,12 @@ block0(v0: i8):
 ; run: %srem_imm_i8(-19) == -1
 ; run: %srem_imm_i8(0xC0) == -1
 ; run: %srem_imm_i8(0x80) == -2
+
+function %srem_with_bmask(i64, i8) -> i8 {
+block0(v0: i64, v1: i8):
+    v2 = bmask.i8 v0
+    v3 = srem v2, v1
+    return v3
+}
+; run: %srem_with_bmask(4352, -1) == 0
+; run: %srem_with_bmask(4352, 1) == 0