possible optimization: convert ui2fp into a select instr if i1 type is used

[llvmbot]

Bugzilla Link	11259
Version	trunk
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@efriedma-quic

Extended Description

when calling si2fp or ui2fp and the input number is of type i1, it is possible to convert it to a select (i1 %c, 0, -1).

[efriedma-quic]

Sure, but why would anyone write code like that?

[llvmbot]

Eli, I came across this code when working on a user bug. I am not sure how important/common this code is. In any case, its better to document this.

[llvmbot]

We could also optimize zext and sext of i1 inputs into the select instruction.
I noticed that ISPC generates these patterns.

For example:
https://github.com/ispc/ispc/blob/master/expr.cpp#L6363 and
https://github.com/ispc/ispc/blob/master/expr.cpp#L6267

[llvmbot]

I fixed this for non-vector types in r159957 and r160036. Doing it for vector types seems more likely to be target-specific, since not all targets have fast vectorized selects.

[arsenm]

This is done but the result looks better if I remove the vector restriction for aarch64