[patch][pr22947] Use sext in fast isel

[Rafael Espíndola]

Fast isel currently has a bug in that it zero extends immediates to 64
bits. This normally goes unnoticed because the value is truncated to
32 bits for output.

Two cases were it is noticed:

* We fail to use smaller encodings.
* If the original constant was smaller than i32.

For example, the only possible values for i1 are 0 and -1, yet we were
expanding -1 to i32 1.

Cheers,
Rafael
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