[mips] Fix some UB by shifting before sign-extending

Avoid shifting a negative value by sign-extending after the shift.

Fixes a couple of tests that were failing under ubsan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240381 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed