Make 91378 more conservative.

1. Only perform (zext (shl (zext x), y)) -> (shl (zext x), y) when y is a constant. This makes sure it remove at least one zest.
2. If the shift is a left shift, make sure the original shift cannot shift out bits.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91399 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/X86/zext-shl.ll

@@ -23,16 +23,3 @@ entry:
   %2 = zext i16 %1 to i32
   ret i32 %2
 }
-
-define i32 @t3(i8 zeroext %x, i8 zeroext %y) nounwind readnone ssp {
-entry:
-; CHECK: t3:
-; CHECK: shll
-; CHECK-NOT: movzwl
-; CHECK: ret
-  %0 = zext i8 %x to i16
-  %1 = zext i8 %y to i16
-  %2 = shl i16 %0, %1
-  %3 = zext i16 %2 to i32
-  ret i32 %3
-}