cmd/gc: remove mentions of "ideal" from error messages.

cmd/gc: remove mentions of "ideal" from error messages.
        _ = complex("foo", 0)
        _ = complex(true, 0)
now trigger:
        x.go:4: invalid operation: complex("foo", 0) (mismatched types untyped string and untyped number)
        x.go:5: invalid operation: complex(true, 0) (mismatched types untyped bool and untyped number)

Fixes #4521

R=golang-dev, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/12973043