Rollup merge of #133452 - taiki-e:hexagon-asm-pred, r=Amanieu

Support predicate registers (clobber-only) in Hexagon inline assembly

The result of the Hexagon instructions such as comparison, store conditional, etc. is stored in predicate registers (`p[0-3]`), but currently there is no way to mark it as clobbered in `asm!`.

This is also needed for `clobber_abi` (although implementing `clobber_abi` will require the addition of support for [several more register classes](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/Hexagon/HexagonRegisterInfo.cpp#L71-L90). see also https://github.com/rust-lang/rust/issues/93335#issuecomment-2395210055).

Refs:
- [Section 6 "Conditional Execution" in Qualcomm Hexagon V73 Programmer’s Reference Manual](https://docs.qualcomm.com/bundle/publicresource/80-N2040-53_REV_AB_Qualcomm_Hexagon_V73_Programmers_Reference_Manual.pdf#page=90)
- [Register definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/Hexagon/HexagonRegisterInfo.td#L155)

cc `@androm3da` (target maintainer of hexagon-unknown-{[none-elf](https://doc.rust-lang.org/nightly/rustc/platform-support/hexagon-unknown-none-elf.html#target-maintainers),[linux-musl](https://doc.rust-lang.org/nightly/rustc/platform-support/hexagon-unknown-linux-musl.html#target-maintainers)})

r? `@Amanieu`

`@rustbot` label +A-inline-assembly
(Currently there is no O-hexagon label...)

compiler/rustc_codegen_llvm/src/asm.rs

@@ -645,6 +645,7 @@ fn reg_to_llvm(reg: InlineAsmRegOrRegClass, layout: Option<&TyAndLayout<'_>>) ->
             | Arm(ArmInlineAsmRegClass::qreg_low4) => "x",
             Arm(ArmInlineAsmRegClass::dreg) | Arm(ArmInlineAsmRegClass::qreg) => "w",
             Hexagon(HexagonInlineAsmRegClass::reg) => "r",
+            Hexagon(HexagonInlineAsmRegClass::preg) => unreachable!("clobber-only"),
             LoongArch(LoongArchInlineAsmRegClass::reg) => "r",
             LoongArch(LoongArchInlineAsmRegClass::freg) => "f",
             Mips(MipsInlineAsmRegClass::reg) => "r",
@@ -813,6 +814,7 @@ fn dummy_output_type<'ll>(cx: &CodegenCx<'ll, '_>, reg: InlineAsmRegClass) -> &'
         | Arm(ArmInlineAsmRegClass::qreg_low8)
         | Arm(ArmInlineAsmRegClass::qreg_low4) => cx.type_vector(cx.type_i64(), 2),
         Hexagon(HexagonInlineAsmRegClass::reg) => cx.type_i32(),
+        Hexagon(HexagonInlineAsmRegClass::preg) => unreachable!("clobber-only"),
         LoongArch(LoongArchInlineAsmRegClass::reg) => cx.type_i32(),
         LoongArch(LoongArchInlineAsmRegClass::freg) => cx.type_f32(),
         Mips(MipsInlineAsmRegClass::reg) => cx.type_i32(),