[LoopVectorizer, TTI] New method supportsEfficientVectorElementLoadStore()

Since SystemZ supports vector element load/store instructions, there is no
need for extracts/inserts if a vector load/store gets scalarized.

This patch lets Target specify that it supports such instructions by means of
a new TTI hook that defaults to false.

The use for this is in the LoopVectorizer getScalarizationOverhead() method,
which will with this patch produce a smaller sum for a vector load/store on
SystemZ.

New test: test/Transforms/LoopVectorize/SystemZ/load-store-scalarization-cost.ll

Review: Adam Nemet
https://reviews.llvm.org/D30680

llvm-svn: 300056