[FuncSpec] Don't specialize function which are easy to inline

It would waste time to specialize a function which would inline finally.
This patch did two things:

- Don't specialize functions which are always-inline.
- Don't spescialize functions whose lines of code are less than threshold
(100 by default).

For spec2017int, this patch could reduce the number of specialized
functions by 33%. Then the compile time didn't increase for every
benchmark.

Reviewed By: SjoerdMeijer, xbolva00, snehasish

Differential Revision: https://reviews.llvm.org/D107897