go/ssa: allow transient untyped integer values

[cpunion]

Non-constant untyped integer values may arise transiently while
evaluating shift expressions. The builder already preserves these values,
but the sanity checker rejects them as invalid SSA.

Permit untyped integer instruction results and operands, while retaining
the checks for all other non-constant untyped values. Add examples that
exercise nested, arithmetic, and untyped-rune shift counts.

This follows the conclusion in golang/go#77067 and the specification
clarification tracked by golang/go#77117.

Fixes golang/go#77067.

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Message from Robert Griesemer:

Patch Set 1: Code-Review+1

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Message from Robert Griesemer:

Patch Set 1: TryBot-Bypass+1

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Message from Robert Griesemer:

Patch Set 1:

(1 comment)

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Message from Jie Li:

Patch Set 1:

(1 comment)

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Message from Robert Griesemer:

Patch Set 1: -TryBot-Bypass

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Message from Robert Griesemer:

Patch Set 1: Commit-Queue+1

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Patch Set 1:

Dry run: CV is trying the patch.

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Patch Set 1: -Commit-Queue

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Patch Set 1: LUCI-TryBot-Result+1

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Message from Robert Griesemer:

Patch Set 1:

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