cmd/compile: ppc64le: sign extension issue in go 1.21rc2 [1.20 backport]

[gopherbot]

@pmur requested issue #61297 to be considered for backport to the next 1.20 minor release.

@gopherbot please consider a backport for 1.20 and 1.19, this is a compiler bug without a workaround.

[gopherbot]

Change https://go.dev/cl/509195 mentions this issue: [release-branch.go1.20] cmd/compile: on PPC64, fix sign/zero extension when masking

[gopherbot]

Closed by merging 6211a02 to release-branch.go1.20.