cmd/compile: compiler crashes in ssa: isNonNegative bad type [1.15 backport]

[gopherbot]

@josharian requested issue #40746 to be considered for backport to the next 1.15 minor release.

@gopherbot please open a backport issue for 1.15. This is a compiler crash on valid input, with no obvious workaround.

[FiloSottile]

Retargeted to Go 1.15.

[gopherbot]

Change https://golang.org/cl/248401 mentions this issue: cmd/compile: correct type of CvtBoolToUint8 values

[dmitshur]

Approved per discussion in a release meeting. Only 1.15 needs this backport.

[gopherbot]

Closed by merging 9c17883 to release-branch.go1.15.