Implement and_sext.ll:test3, generating:

Implement and_sext.ll:test3, generating:

_test4:
        srawi r3, r3, 16
        blr

instead of:

_test4:
        srwi r2, r3, 16
        extsh r3, r2
        blr

for:

short test4(unsigned X) {
  return (X >> 16);
}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28174 91177308-0d34-0410-b5e6-96231b3b80d8