[LoopVectorizer] Refine loop vectorizer's register usage calculator by ignoring speci...

[LoopVectorizer] Refine loop vectorizer's register usage calculator by ignoring specific instructions.

LoopVectorizationCostModel::calculateRegisterUsage() is used to estimate the
register usage for specific VFs. However, it takes into account many
instructions that won't be vectorized, such as induction variables,
GetElementPtr instruction, etc.. This makes the loop vectorizer too conservative
when choosing VF. In this patch, the induction variables that won't be
vectorized plus GetElementPtr instruction will be added to ValuesToIgnore set
so that their register usage won't be considered any more.

Differential revision: http://reviews.llvm.org/D15177

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255454 91177308-0d34-0410-b5e6-96231b3b80d8