Fix AArch64InlineAsmReg::emit

[taiki-e]

Currently, this method uses self as u32 - Self::x0 as u32 to get register index:

rust/compiler/rustc_target/src/asm/aarch64.rs

Line 204 in 3678036

(modifier.unwrap_or('x'), self as u32 - Self::x0 as u32)

However, this is incorrect for reasons explained in the following comment:

rust/compiler/rustc_codegen_llvm/src/asm.rs

Lines 544 to 549 in 3678036

	fn a64_reg_index(reg: InlineAsmReg) -> Option<u32> {
	use AArch64InlineAsmReg::*;
	// Unlike `a64_vreg_index`, we can't subtract `x0` to get the u32 because
	// `x19` and `x29` are missing and the integer constants for the
	// `x0`..`x30` enum variants don't all match the register number. E.g. the
	// integer constant for `x18` is 18, but the constant for `x20` is 19.

r? @Amanieu

@rustbot label O-AArch64 +A-inline-assembly

[Amanieu]

@bors r+

[bors]

📌 Commit 67ebb6c has been approved by Amanieu

It is now in the queue for this repository.