[X86][SSE] Reapplied: Simplify vector LOAD + EXTEND on pre-SSE41 hardware
Improve vector extension of vectors on hardware without dedicated VSEXT/VZEXT instructions.
We already convert these to SIGN_EXTEND_VECTOR_INREG/ZERO_EXTEND_VECTOR_INREG but can further improve this by using the legalizer instead of prematurely splitting into legal vectors in the combine as this only properly helps for lowering to VSEXT/VZEXT.
Removes a lot of unnecessary any_extend + mask pattern - (Fix for PR25718).
Reapplied with a fix for PR26953 (missing vector widening legalization).
Differential Revision: http://reviews.llvm.org/D17932
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264062
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