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_target/src/asm/hexagon.rs

@@ -7,6 +7,7 @@ use super::{InlineAsmArch, InlineAsmType, ModifierInfo};
 def_reg_class! {
     Hexagon HexagonInlineAsmRegClass {
         reg,
+        preg,
     }
 }
 
@@ -37,6 +38,7 @@ impl HexagonInlineAsmRegClass {
     ) -> &'static [(InlineAsmType, Option<Symbol>)] {
         match self {
             Self::reg => types! { _: I8, I16, I32, F32; },
+            Self::preg => &[],
         }
     }
 }
@@ -71,6 +73,10 @@ def_regs! {
         r26: reg = ["r26"],
         r27: reg = ["r27"],
         r28: reg = ["r28"],
+        p0: preg = ["p0"],
+        p1: preg = ["p1"],
+        p2: preg = ["p2"],
+        p3: preg = ["p3"],
         #error = ["r19"] =>
             "r19 is used internally by LLVM and cannot be used as an operand for inline asm",
         #error = ["r29", "sp"] =>