Implement AsmParser for ARM unwind directives.

This commit implements the AsmParser for fnstart, fnend,
cantunwind, personality, handlerdata, pad, setfp, save, and
vsave directives.

This commit fixes some minor issue in the ARMELFStreamer:

* The switch back to corresponding section after the .fnend
  directive.

* Emit the unwind opcode while processing .fnend directive
  if there is no .handlerdata directive.

* Emit the unwind opcode to .ARM.extab while processing
  .handlerdata even if .personality directive does not exist.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181603 91177308-0d34-0410-b5e6-96231b3b80d8
InstCombine: Don't claim to be able to evaluate any shl in a zexted type.

The shift amount may be larger than the type leading to undefined behavior.
Limit the transform to constant shift amounts. While there update the bits to
clear in the result which may enable additional optimizations.

PR15959.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181604 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181606 91177308-0d34-0410-b5e6-96231b3b80d8