[X86] Improve mul combine for negative multiplayer (2^c - 1)

This patch improves the mul instruction combine function (combineMul) 
by adding new layer of logic. 
In this patch, we are adding the ability to fold (mul x, -((1 << c) -1)) 
or (mul x, -((1 << c) +1)) into (neg(X << c) -x) or (neg((x << c) + x) respective.

Differential Revision: https://reviews.llvm.org/D28232

llvm-svn: 292358