[X86][Asm] Allow negative immediate to appear before bracketed expression

Currently, only non-negative immediate is allowed prior to a brac expression (memory reference).
MASM / GAS does not have any problem cope with the left side of the real line, so we should be able to as well.

Differntial Revision: https://reviews.llvm.org/D36229



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310528 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed