[CGP] Relax a bit restriction for optimizeMemoryInst to extend scope

CodeGenPrepare::optimizeMemoryInst contains a check that we do nothing
if all instructions combining the address for memory instruction is in the same
block as memory instruction itself.

However if any of these instruction are placed after memory instruction then
address calculation will not be folded to memory instruction.

The added test case shows an example.

Reviewers: loladiro, spatel, efriedma
Reviewed By: efriedma
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D34862

llvm-svn: 307628