cmd/compile/internal/ir: add missing SetTypecheck

This was missed earlier, because NewConstAt is only used now to
construct the predeclared "true" and "false" constants. But these
constants are no longer actually accessed with unified IR.

For constant expressions, types2 (and go/types) sets
TypeAndValue.Value for the expression to the appropriate constant
value. The unified writer recognizes when expressions are constants,
and simply writes the underlying value, regardless of the original
expression. As a result, we never end up actually referencing the
*named* "true" and "false" constants; we just always construct
anonymous constant "true" and "false" values.

However, a manually constructed tree that includes an *ir.Name that
"Uses" the predeclared true/false Const Objects, yet doesn't set
TypeAndValue.Value will instead end up trying to use named constants
constructed with NewConstAt.

Thanks to Russ for reporting the issue on CL 510541, and to Cuong for
identifying the fix.

Change-Id: I0614105379d63ea76d7244ebd1e4db5c239d4670
Reviewed-on: https://go-review.googlesource.com/c/go/+/524357
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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