microsoft/compiler: do not lower away 64-bit ffma

In DXIL, the FMA instruction only supports 64-bit operations. However,
back when we implemented support for this, there were only a single
switch for lowering all ffma instructions, so we couldn't easily use it.

But now that there's separate flags to lower ffma on 16, 32 and 64 bit,
we can lower 16 and 32 bit ffmas, and leave 64 bit ffmas alone.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <!8349>
37 jobs for !8349 with dxil-fma-cleanup in 8 minutes and 33 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Sanity
passed #6539251
sanity

00:00:18

 
  Container
passed #6539256
aarch64
arm_build

00:00:20

passed #6539258
arm_test-base

00:00:18

passed #6539260
windows shell 1809 mesa
windows_build_vs2019

00:00:18

passed #6539253
x86_build-base

00:00:17

passed #6539255
x86_test-base

00:00:12

 
  Container 2
passed #6539275
android_build

00:00:16

passed #6539280
arm64_test

00:00:12

passed #6539282
armhf_test

00:00:16

passed #6539270
i386_build

00:00:16

passed #6539262
kernel+rootfs_amd64

00:00:15

passed #6539264
aarch64
kernel+rootfs_arm64

00:01:01

passed #6539266
aarch64
kernel+rootfs_armhf

00:00:18

passed #6539272
ppc64el_build

00:00:11

passed #6539273
s390x_build

00:00:15

passed #6539268
x86_build

00:00:17

passed #6539277
x86_test-gl

00:00:20

passed #6539278
x86_test-vk

00:00:19

 
  Meson X86 64
passed #6539292
meson-clang

00:02:32

passed #6539294
meson-clover

00:02:22

passed #6539287
meson-clover-testing

00:01:03

passed #6539289
meson-gallium

00:03:44

passed #6539291
meson-release

00:00:59

passed #6539284
meson-testing

00:02:02

passed #6539285
meson-testing-asan

00:07:07

passed #6539296
meson-vulkan

00:01:51

 
  Build Misc
passed #6539298
meson-android

00:01:40

passed #6539301
aarch64
meson-arm64

00:01:15

passed #6539303
aarch64
meson-arm64-build-test

00:01:18

passed #6539299
aarch64
meson-armhf

00:01:19

passed #6539309
meson-i386

00:01:53

passed #6539315
meson-mingw32-x86_64

00:01:24

passed #6539313
kvm
meson-ppc64el

00:03:30

passed #6539311
kvm
meson-s390x

00:03:15

passed #6539305
windows docker 1809 mesa
meson-windows-vs2019

00:04:05

passed #6539307
scons-win64

00:03:10

 
  Layered Backends
passed #6539317
windows docker 1809 mesa
test-d3d12-windows

00:03:40