cmd/compile: generic shift-by-constant canonicalization gives Const64 the wrong type [1.26 backport]

@Jorropo

@Jorropo requested issue #81868 to be considered for backport to the next 1.26 minor release.

cc @gopherbot please open a backport issue for 1.26

This causes me confirmed crashes, maybe misscompilations too.

@Jorropo

This is a request to backport https://go.dev/cl/795240.

IIUC, #81706 is a 1.27 backport for a regression caused by https://go.dev/cl/795240.

Thus if this request is approved, I believe we need to backport:

@Jorropo is this corrrect?

@prattmic

There are three bugs here:

Const64 typed with the underlying rotate, which can lead to Const64 with type of 32 16 or 8 bits.
Const32 typed with the underlying rotate on 32 bits platforms, which can lead to Const32 with type 16 or 8 bits.
Const32 typed with the previous Const64 type when canonicalizing 64bits rotates on 32 bits platforms.

CL 795240 fixes 1 and 2 but it introduces 3.
CL 837245 fixes 3.

Approving the backport to Go 1.26 for CL 795240 and CL 837245.

Change https://go.dev/cl/845965 mentions this issue: [release-branch.go1.26] cmd/compile: fix misscompile in rewrite generic rules

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