asm: Add a kreg0 register class on x86 which includes k0

Previously we only exposed a kreg register class which excludes the k0
register since it can't be used in many instructions. However k0 is a
valid register and we need to have a way of marking it as clobbered for
clobber_abi.

Fixes rust-lang#94977

Author: Amanieu

src/asm.rs

@@ -581,6 +581,7 @@ fn reg_to_gcc(reg: InlineAsmRegOrRegClass) -> ConstraintOrRegister {
             | InlineAsmRegClass::X86(X86InlineAsmRegClass::ymm_reg) => "x",
             InlineAsmRegClass::X86(X86InlineAsmRegClass::zmm_reg) => "v",
             InlineAsmRegClass::X86(X86InlineAsmRegClass::kreg) => unimplemented!(),
+            InlineAsmRegClass::X86(X86InlineAsmRegClass::kreg0) => unimplemented!(),
             InlineAsmRegClass::Wasm(WasmInlineAsmRegClass::local) => unimplemented!(),
             InlineAsmRegClass::X86(
                 X86InlineAsmRegClass::x87_reg | X86InlineAsmRegClass::mmx_reg,
@@ -646,6 +647,7 @@ fn dummy_output_type<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, reg: InlineAsmRegCl
         | InlineAsmRegClass::X86(X86InlineAsmRegClass::zmm_reg) => cx.type_f32(),
         InlineAsmRegClass::X86(X86InlineAsmRegClass::x87_reg) => unimplemented!(),
         InlineAsmRegClass::X86(X86InlineAsmRegClass::kreg) => cx.type_i16(),
+        InlineAsmRegClass::X86(X86InlineAsmRegClass::kreg0) => cx.type_i16(),
         InlineAsmRegClass::Wasm(WasmInlineAsmRegClass::local) => cx.type_i32(),
         InlineAsmRegClass::SpirV(SpirVInlineAsmRegClass::reg) => {
             bug!("LLVM backend does not support SPIR-V")
@@ -760,6 +762,7 @@ fn modifier_to_gcc(arch: InlineAsmArch, reg: InlineAsmRegClass, modifier: Option
             _ => unreachable!(),
         },
         InlineAsmRegClass::X86(X86InlineAsmRegClass::kreg) => None,
+        InlineAsmRegClass::X86(X86InlineAsmRegClass::kreg0) => None,
         InlineAsmRegClass::X86(X86InlineAsmRegClass::x87_reg | X86InlineAsmRegClass::mmx_reg) => {
             unreachable!("clobber-only")
         }