[LV] Avoid rounding errors for predicated instruction costs

This patch modifies the cost calculation of predicated instructions (div and
rem) to avoid the accumulation of rounding errors due to multiple truncating
integer divisions. The calculation for predicated stores will be addressed in a
follow-on patch since we currently don't scale the cost of predicated stores by
block probability.

Differential Revision: https://reviews.llvm.org/D25333

llvm-svn: 284123