[CodeGen] refactor getDwarfRegNum()

[chenzheng1030]

This is from code review of #95837 (comment)

We should use the getDwarfRegNum() defined in MCRegisterInfo.h instead of defining a new one in llvm/lib/CodeGen/StackMaps.cpp

The DwarfRegAlias may not be needed if we implement the getDwarfRegNum() by sharing the DWARF reg numbers among sub/supper registers.

[llvmbot]

@llvm/issue-subscribers-debuginfo

Author: Chen Zheng (chenzheng1030)

This is from code review of https://github.com//pull/95837#discussion_r1643647221

We should use the getDwarfRegNum() defined in MCRegisterInfo.h instead of defining a new one in llvm/lib/CodeGen/StackMaps.cpp

The DwarfRegAlias may not be needed if we implement the getDwarfRegNum() by sharing the DWARF reg numbers among sub/supper registers.