[CGP] Make optimizeMemoryInst able to combine more kinds of ExtAddrMode fields

This patch extends the recent work in optimizeMemoryInst to make it able to
combine more ExtAddrMode fields than just the BaseReg.

This fixes some benchmark regressions introduced by r309397, where GVN PRE is
hoisting a getelementptr such that it can no longer be combined into the
addressing mode of the load or store that uses it.

Differential Revision: https://reviews.llvm.org/D38133

llvm-svn: 318949