[release-branch.go1.10] cmd/compile: fix zero extend after float->int conversion

[release-branch.go1.10] cmd/compile: fix zero extend after float->int conversion

Don't do direct loads from argument slots if the sizes don't match.
This prevents us from loading from a float32 using a uint64 load
during expressions like uint64(math.float32Bits(f)) where f is a float32 arg.

Fixes #25335

Change-Id: I3887d76f78c844ba546243e7721d811c3d4a9700
Reviewed-on: https://go-review.googlesource.com/112637
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/131276
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>