[ConstantHoisting] remove a LLVM_DEBUG statement

There is no need to print the entire function after a transform via
LLVM_DEBUG statements.  These can be emulated via:
$ llc -print-after=consthoist -filter-print-funcs=<function name>

Otherwise, this makes the output of
$ llc -debug-only=consthoist
too verbose.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D154904