[ARM] Add support for the X asm constraint

Summary:
This patch adds support for the X asm constraint.

To do this, we lower the constraint to either a "w" or "r" constraint
depending on the operand type (both constraints are supported on ARM).

Fixes PR26493

Reviewers: t.p.northover, echristo, rengolin

Subscribers: joker.eph, jgreenhalgh, aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D19061

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267411 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed