Regression with r200947: [CodeGenPrepare] Move away sign extensions that get in the way of addressing mode.

[Tom Stellard]

Hi Quentin,

I've discovered a regression with this commit, please see the attached test case.
In this case, CodeGenPrepare is promoting the mul in a sext + mul pattern even
though the addressing mode it is creating isn't legal.

One interesting thing about this test case is that if you remove the
nsw from the mul instruction, then the incorrect transform does not
take place.  I'm not sure why this matters.  Would you mind taking a look?

Thanks,
Tom
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; RUN: opt -codegenprepare -S -o - %s | FileCheck %s

target datalayout = "e-p:32:32-p1:64:64-p2:64:64-p3:32:32-p4:32:32-p5:64:64-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-v2048:2048-n32:64"
target triple = "r600--"

; CHECK-LABEL: @test
; CHECK: mul
; CHECK-NEXT: sext
define void @test(i8 addrspace(1)* nocapture readonly %in, i32 %a, i8 %b) {
entry:
  %0 = mul nsw i32 %a, 3
  %1 = sext i32 %0 to i64
  %2 = getelementptr i8 addrspace(1)* %in, i64 %1
  store i8 %b, i8 addrspace(1)* %2
  ret void
}