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

llvm: D36229

Differential Revision: https://reviews.llvm.org/D36230


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@310529 91177308-0d34-0410-b5e6-96231b3b80d8

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