[PATCH] [InstCombine] Canonicalize addrspacecast between different element types

[Jingyue Wu]

Hi arsenm, eliben, meheff,

addrspacecast X addrspace(M)* to Y addrspace(N)*
-->
bitcast X addrspace(M)* to Y addrspace(M)*
addrspacecast Y addrspace(M)* to Y addrspace(N)*

This canonicalization makes NVPTXFavorNonGenericAddrSpaces more effective, and can potentially benefit many other cases. 

This patch is based on D2186 with fixes and more tests:
* Fix an issue in D2186 that causes InstCombine to run into infinite loops
* Add the failed test as @canonicalize_addrspacecast in test/Transforms/InstCombine/addrspacecast.ll
* Do bitcast before addrspacecast because addrspacecasts from non-generic to generic can be folded into load/store
* Updated all affected tests. One affected test (@test2_addrspacecast) in memcpy-from-global.ll is actually better optimized because of this canonicalization. See how alloca %T being transformed to alloca [128 x i8]

http://reviews.llvm.org/D4013

Files:
  lib/IR/Instructions.cpp
  lib/Transforms/InstCombine/InstCombineCasts.cpp
  test/Transforms/InstCombine/addrspacecast.ll
  test/Transforms/InstCombine/getelementptr.ll
  test/Transforms/InstCombine/memcpy-from-global.ll
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