Recommit "[InlineCost] Use TTI to check if GEP is free."

This recommits r292526 which is reverted in r292529 after fixing the test case.

The original summary:

Currently, a GEP is considered free only if its indices are all constant.
TTI::getGEPCost() can give target-specific more accurate analysis. TTI is
already used for the cost of many other instructions.

llvm-svn: 292570