[X86TTI] Make constant base pointers for getElementPtr opaque.

[X86TTI] Make constant base pointers for getElementPtr opaque.

If getElementPtr uses a constant as base pointer, then make the constant opaque.
This prevents constant folding it with the offset. The offset can usually be
encoded in the load/store instruction itself and the base address doesn't have
to be rematerialized several times.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204739 91177308-0d34-0410-b5e6-96231b3b80d8