When matching asm operands, always try to match the most restricted type first.

Unfortunately, while this is the "right" thing to do, it breaks some ARM
asm parsing tests because MemMode5 and ThumbMemModeReg are ambiguous.  This
is tricky to resolve since neither is a subset of the other.

XFAIL the test for now.  The old way was broken in other ways, just ways
we didn't happen to be testing, and our ARM asm parsing is going to require
significant revisiting at a later point anyways.

llvm-svn: 123786