[InlineCost] Enable the new switch cost heuristic

Summary:
This is to enable the new switch inline cost heuristic (r301649) by removing the
old heuristic as well as the flag itself.
In my experiment for LLVM test suite and spec2000/2006, +17.82% performance and
8% code size reduce was observed in spec2000/vertex with O3 LTO in AArch64.
No significant code size / performance regression was found in O3/O2/Os. No
significant complain was reported from the llvm-dev thread.

Reviewers: hans, chandlerc, eraman, haicheng, mcrosier, bmakam, eastig, ddibyend, echristo

Reviewed By: echristo

Subscribers: javed.absar, kristof.beyls, echristo, aemerson, rengolin, mehdi_amini

Differential Revision: https://reviews.llvm.org/D32653

llvm-svn: 304594