[x86] Allow merging multiple instances of an immediate within a basic block...

[x86] Allow merging multiple instances of an immediate within a basic block for code size savings, for 64-bit constants.

This patch handles 64-bit constants which can be encoded as 32-bit immediates.

It extends the functionality added by https://reviews.llvm.org/D11363 for 32-bit constants to 64-bit constants.

Patch by Sunita Marathe!

Differential Revision: https://reviews.llvm.org/D23391

llvm-svn: 278857