[mips] Improve vr4300 mulmul bugfix pass

When compiling with dwarf info, the mfix4300 flag introduced in
https://reviews.llvm.org/D116238 can miss some occurrences of the vr4300
mulmul bug if a debug instruction happens to be between two `muls`
instructions. This change skips debug instructions in order to fix
the mulmul bug detection.

Fixes https://github.com/llvm/llvm-project/issues/53094

Differential Revision: https://reviews.llvm.org/D117615