[SCEV] Compute AddRec range computations using different type BECount

Before this patch, we can only use the MaxBECount for an AddRec's range
computation if the MaxBECount has <= bit width of the AddRec. This patch
reasons that if a MaxBECount has > bit width, and is <= the max value of
AddRec's bit width, we can still use the MaxBECount.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D151698