Revert "Fix overly conservative inlining prediction".

The commit itself should be fine, but the more aggressive
inlining being done on the GC code (due to the gcbits functions
being in available in gc.gcx) seems to have uncovered a
misoptimization bug in LLVM (at least in 3.2) on x86, leading to
various unit tests failing in relase mode.

This reverts part of commit f02e4b1925f17186aed5b543e3b02a857da7b14b.