[PATCH] [CodeGen] Annotate signed shift lefts with NUW

[David Majnemer]

In http://reviews.llvm.org/D8899#153339, @chandlerc wrote:

> This doesn't seem correct for a negative LHS where we only shift out 1s -- I think that should be valid.
>
> Essentially, we seem to have a strange circumstance here where NUW as specified is not a subset of the restrictions of NSW.


Negative LHS is forbidden in both C11 and C++14.

C11 6.5.7p4:
If E1 has a signed type and nonnegative value, and E1 × 2**E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

C++14 [expr.shift]p2:
Otherwise, if E1 has a signed type and non-negative value, and E1 × 2**E2 is representable in the corresponding unsigned type of the result type, then that value, converted to the result type, is the resulting value; otherwise, the behavior is undefined.


http://reviews.llvm.org/D8899

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