AArch64: allow constant expressions for shifted reg literals

e.g., add w1, w2, w3, lsl #(2 - 1)

This sort of thing comes up in pre-processed assembly playing macro games.
Still validate that it's an assembly time constant. The early exit error check
was just a bit overzealous and disallowed a left paren.

rdar://18430542

llvm-svn: 218336