There are 4 distinct cases of fsqrt:

1. FP16 on Bifrost

Here we may use lower to `x * rsqrt(x)` with a .left modifier on the
FMA_RSCALE.v2f16 used to carry out the multiplication, ensuring correct
handling of NaN and Inf.

2. FP32 on G71

Missing FRSQ.f32 instruction, do something simple since we don't even
probe the driver on G71...

3. FP32 on G72 and newer

We can do the same lowering as FP16 in theory. However, this may have
precision issues. The DDK uses extra FREXPM/FREXPE instructions in a
.sqrt mode for a range reduction. It's unknown if this is necessary for
OpenGL (ES), Vulkan, OpenCL, or some combination thereof.

4. FP16 on Valhall

We want to use the same strategy as on Bifrost, but Valhall removed the
FMA_RSCALE.v2f16 instruction. Instead we use an ordinary FMA.v2f16 for
the multiply and check the special case explicitly with CSEL.v2f32 ...
I'm not sure if this is right for infinity but the DDK does it so ¯\_(ツ)_/¯

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>