[X86] Fix crash with inline asm using wrong register name

Fixes PR#48678. `X86TargetLowering::getRegForInlineAsmConstraint()` can adjust the register class to match the type, e.g. change `VR128X` to `VR256X` if the type needs 256 bits. However, the function currently returns the unadjusted register and the adjusted register class, e.g. `xmm15` and `VR256X`, which then causes an assertion failure later because the register class does not contain that register. This patch fixes this behavior.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D113834