[dev.simd] simd, cmd/compile: add float -> float conversions

[dev.simd] simd, cmd/compile: add float -> float conversions

This should mark the end of the conversion table, except for float16
which does not exist on Go yet.

The rounding logic documentation of float64 -> float32 is based on
abi-internal default MXCSR:
| RC | 14/13 | 0 (RN) | Round to nearest |

Change-Id: I27a86560e8d74d20f21350bf78314b4eada20ec0
Reviewed-on: https://go-review.googlesource.com/c/go/+/724440
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>