[TargetLowering] StringRefize asm constraint getters.

[TargetLowering] StringRefize asm constraint getters.

There is some functional change here because it changes target code from
atoi(3) to StringRef::getAsInteger which has error checking. For valid
constraints there should be no difference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241411 91177308-0d34-0410-b5e6-96231b3b80d8