Merge c88beb4112d5bbf07d76a615ab7f13ba2ba023e6 into 18.x

[brad0]

MIPS: Fix asm constraints "f" and "r" for softfloat (#79116)

This include 2 fixes:
1. Disallow 'f' for softfloat.
2. Allow 'r' for softfloat.

Currently, 'f' is accpeted by clang, then LLVM meets an internal error.

'r' is rejected by LLVM by: couldn't allocate input reg for constraint
'r'.

Fixes: #64241, #63632

[brad0]

/cherry-pick c88beb4

[llvmbot]

/pull-request #83105

[llvmbot]

@llvm/issue-subscribers-backend-mips

Author: Brad Smith (brad0)

MIPS: Fix asm constraints "f" and "r" for softfloat (#79116)

This include 2 fixes:
1. Disallow 'f' for softfloat.
2. Allow 'r' for softfloat.

Currently, 'f' is accpeted by clang, then LLVM meets an internal error.

'r' is rejected by LLVM by: couldn't allocate input reg for constraint
'r'.

Fixes: #64241, #63632