[X86] Allow merging of immediates within a basic block for code size savings

First step in preventing immediates that occur more than once within a single
basic block from being pulled into their users, in order to prevent unnecessary
large instruction encoding .Currently enabled only when optimizing for size.

Patch by: zia.ansari@intel.com
Differential Revision: http://reviews.llvm.org/D11363

llvm-svn: 244601