[CGP] Disable Select instruction handling in optimizeMemoryInst. NFC

This patch disables the handling of selects in optimization
extensing scope of optimizeMemoryInst.

The optimization itself is disable by default.
The idea here is just to switch optimiztion level step by step.

Specifically, first optimization will be enabled only for Phi nodes,
then select instructions will be added.

In case someone will complain about perfromance it will be easier to
detect what part of optimizations is responsible for that.

Differential Revision: https://reviews.llvm.org/D36073

llvm-svn: 317555