Machine supports (or the runtime simulates in soft float mode)
(u)int32<->float conversions. The frontend rewrites int64<->float
conversions to call to runtime function.

For int64->float32 conversion, the frontend generates

.   .   AS u(100) l(10) tc(1)
.   .   .   NAME-main.~r1 u(1) a(true) g(1) l(9) x(8+0) class(PPARAMOUT) f(1) float32
.   .   .   CALLFUNC u(100) l(10) tc(1) float32
.   .   .   .   NAME-runtime.int64tofloat64 u(1) a(true) x(0+0) class(PFUNC) tc(1) used(true) FUNC-func(int64) float64

The CALLFUNC node has type float32, whereas runtime.int64tofloat64
returns float64. The legacy backend implicitly makes a float64->float32
conversion. The SSA backend does not do implicit conversion, so we
insert an explicit CONV here.

All cmd/compile/internal/gc/testdata/*_ssa.go tests passed.

Progress on SSA for ARM. Still not complete.

Update #15365.

Change-Id: I30937c8ff977271246b068f48224693776804339
Reviewed-on: https://go-review.googlesource.com/23652Reviewed-by: Keith Randall <khr@golang.org>