PATCH for PR18045 (X86 inst selection failure)

[Stepan Dyatkovskiy]

Hello,

Please find fix + test-case in attachment.

Short description of PR18045 issue:
For X86 machines with sse < sse4.1 we got failures for some
particular load/store vector sequences:

$ clang-trunk -m32 -O2 test-case.c
fatal error: error in backend: Cannot select: 0x4200920: v4i32,ch = load 
0x41d6ab0, 0x4205850,
       0x41dcb10<LD16[getelementptr inbounds ([4 x i32]* @e, i32 0, i32 
0)](align=4)> [ORD=82]
       [ID=58]
   0x4205850: i32 = X86ISD::Wrapper 0x41d5490 [ORD=26] [ID=43]
     0x41d5490: i32 = TargetGlobalAddress<[4 x i32]* @e> 0 [ORD=26] [ID=23]
   0x41dcb10: i32 = undef [ID=2]

The reason is that EltsFromConsecutiveLoads could emit such load 
instruction both before and after legalize stage. Though this 
instruction is not legal for machines with SSSE3 and lower.

The fix: In EltsFromConsecutiveLoads, if we have passed legalize stage, 
we check whether nodes it emits are legal.

-Stepan
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