[LoopUnroll] Prevent LoopFullUnrollPass to perform partial/runtime unrolling

FullLoopUnroll was performing runtime unrolling in certain cases when
'#pragma unroll' was specified. Patch to fix this by introducing new parameter
to tryToUnrollLoop() to differentiate between LoopUnrollPass and
FullLoopUnrollPass. Based on the discussion here
(https://discourse.llvm.org/t/loop-unroller-fails-to-unroll-loop/69834)

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D148071