Have the integrated assembler give an error if $1 is used as an identifier in
an expression.  Currently this bug causes the line to be ignored in a
release build and an assert in a debug build.

rdar://13062484


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173195 91177308-0d34-0410-b5e6-96231b3b80d8