[TTI][NFC] Introduce option to set predictable branch threshold

Currently TargetTransformInfo::getPredictableBranchThreshold() method
returns hardcoded value 99. This value affects the decision whether to
convert select instruction to branch or not in several passes:
SelectOptimize, CodeGenPrepare, SimplifyCFG.

It would be useful to make possible to play with that threshold in order
to test select-optimize heuristics.

Option was originally introduced in the TargetLoweringBase, but was
removed in the revision 664d0c05 and not restored in the TTI

Patch Author: aleksandr.popov
Reviewed By: spatel
Differential Revision: https://reviews.llvm.org/D143060