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
