[clang] [llvm] [MIPS][ISel] Fix musttail (PR #161860)

[via llvm-commits]

ameisen wrote:

As a separate problem, with that simple code sample, if I add the `musttail` attribute, with your PR I am getting this error:

`'musttail' attribute for this call is impossible because calls outside the current linkage unit cannot be tail called on MIPS`

https://github.com/llvm/llvm-project/pull/161860