cmd/compile: store constant floats using integer constants
x86 is better at storing constant ints than constant floats.
(It uses a constant directly in the instruction stream, instead of
loading it from a constant global memory.)
Noticed as part of #67957
Change-Id: I9b7b586ad8e0fe9ce245324f020e9526f82b209d
Reviewed-on: https://go-review.googlesource.com/c/go/+/592596
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>