go/types, types2: error reported for valid shifts

[griesemer]

Extracted from test/shift1.go which is currently not passing (excluded) in 1.18:

package p

func _(x int, s uint) {
	x = (1. << s) << (1 << s)
	x = (1. << s) << (1. << s)
}

reports

x.go:4:6: invalid operation: shifted operand (1. << s) (untyped float value) must be integer
x.go:5:6: invalid operation: shifted operand (1. << s) (untyped float value) must be integer

These should probably be valid. Regression.

cc @findleyr

[gopherbot]

Change https://go.dev/cl/398316 mentions this issue: types2: don't report an error for certain valid shifts