cmd/compile/internal/ir: remove un-used code for const

cmd/compile/internal/ir: remove un-used code for const

CL 390474 removed last usages of ConstValue, it can now be removed, and
also Float64Val, since when it's only used by ConstValue.

CanInt64 is un-used for a long time, its original form last usage was
removed in CL 221802.

Change-Id: Id142b0da49c319faca73ef1b2090325f81431321
Reviewed-on: https://go-review.googlesource.com/c/go/+/396078
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>