Fix a bug in the MC asm parser evaluating expressions.  It was treating:
A = 9
B = 3 * A - 2 * A + 1 as  B = 3 * A - (2 * A + 1)

rdar://13816516


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181366 91177308-0d34-0410-b5e6-96231b3b80d8