[SimplifyCFG] Swap to using TargetTransformInfo for cost
We're already using TTI in SimplifyCFG, so remove the hard-baked "cheapness"
heuristic and use TTI directly. Generally NFC intended, but we're using a slightly
different heuristic now so there is a slight test churn.
- combine-comparisons-by-cse.ll: Removed unneeded branch check.
- 2014-08-04-muls-it.ll: Test now doesn't branch but emits muleq.
- coalesce-subregs.ll: Superfluous block check.
- 2008-01-02-hoist-fp-add.ll: fadd is safe to speculate. Change to udiv.
- PhiBlockMerge.ll: Superfluous CFG checking code. Main checks still present.
- select-gep.ll: A variable GEP is not expensive, just TCC_Basic, according to the TTI.