[ConstHoisting] Turn on consthoist-with-block-frequency by default.

Using profile information to guide consthoisting is generally helpful for
performance, so the patch turns it on by default. No compile time or perf
regression were found using spec2000 and spec2006 on x86.  Some significant
improvement (>20%) was seen on internal benchmarks.

Differential Revision: https://reviews.llvm.org/D35063

llvm-svn: 307338