Lower certain build_vectors to insertps instructions

[Demikhovsky, Elena]

I added some comments, please see inside.

-  Elena

-----Original Message-----
From: Filipe Cabecinhas [mailto:filcab+llvm.phabricator at gmail.com] 
Sent: Sunday, April 27, 2014 08:54
To: filcab+llvm.phabricator at gmail.com; nrotem at apple.com; Demikhovsky, Elena; craig.topper at gmail.com
Cc: llvm-commits at cs.uiuc.edu
Subject: [PATCH] Lower certain build_vectors to insertps instructions

Hi nadav, delena, craig.topper,

Vectors built with zeros and elements in the same order as another
(source) vector are optimized to be built using a single insertps instruction.

Further optimizations are possible, described in TODO comments.
I will be implementing at least some of them in the near future.

Added some tests for different cases where this optimization triggers.

http://reviews.llvm.org/D3521

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/sse41.ll
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