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

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

(This is the third attempt to check in this patch, and the first two are r255454
and r255460. The once failed test file reg-usage.ll is now moved to
test/Transform/LoopVectorize/X86 directory with target datalayout and target
triple indicated.)

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@255691 91177308-0d34-0410-b5e6-96231b3b80d8