[CodeGenPrepare] Estimate liveness of loop invariants when checking for...

[CodeGenPrepare] Estimate liveness of loop invariants when checking for address folding profitability

When checking the profitability of folding an address computation
into a memory instruction, the compiler tries to determine the liveness
of the values, comprising the address, at the point of the memory instruction.
This patch improves on the live variable estimates by including
the loop invariants which are references in the loop body.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D143897