ARM: Permit "sp" in ARM variant of STREXD instructions

Patch from Mihail Popa

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179854 91177308-0d34-0410-b5e6-96231b3b80d8
[ms-inline asm] Cleanup the dot operator implementation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179856 91177308-0d34-0410-b5e6-96231b3b80d8