[SelectionDAGBuilder] Simplify creation of shufflevector DAG nodes where...

[SelectionDAGBuilder] Simplify creation of shufflevector DAG nodes where inputs are larger than the mask

Summary:
The current code loops over all elements to calculate a used range. Then a second short loop looks at the ranges and determines if they can be used in a extract and creates a properly aligned start index for the extract.

This range finding is unnecessary, we can just calculate a properly aligned start index for an extract for each input during the first loop. If we don't find the same start index for each indice we can't use an extract.

Reviewers: zvi, RKSimon

Reviewed By: zvi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29926

llvm-svn: 295152