soft-fp64: Simplify __countLeadingZeros32 function

findMSB returns -1 for an input of zero, so 31 - findMSB(a) is
sufficient on its own.

There's only one user of findMSB in shader-db, and it does not match
this pattern.

TODO: Add a pattern in the backend code generator that emits 31 -
nir_op_ufind_msb(a) as if it were nir_op_uclz.  That should save a couple
instructions.

Results on the 308 shaders extracted from the fp64 portion of the OpenGL
CTS:

Tiger Lake and Ice Lake had similar results. (Tiger Lake shown)
total instructions in shared programs: 859509 -> 848109 (-1.33%)
instructions in affected programs: 841058 -> 829658 (-1.36%)
helped: 97
HURT: 0
helped stats (abs) min: 3 max: 1161 x̄: 117.53 x̃: 72
helped stats (rel) min: 0.98% max: 6.74% x̄: 1.70% x̃: 1.35%
95% mean confidence interval for instructions value: -147.21 -87.84
95% mean confidence interval for instructions %-change: -1.94% -1.46%
Instructions are helped.

total cycles in shared programs: 7072275 -> 6969145 (-1.46%)
cycles in affected programs: 6955767 -> 6852637 (-1.48%)
helped: 97
HURT: 0
helped stats (abs) min: 32 max: 10900 x̄: 1063.20 x̃: 560
helped stats (rel) min: 1.18% max: 7.58% x̄: 1.84% x̃: 1.45%
95% mean confidence interval for cycles value: -1339.43 -786.96
95% mean confidence interval for cycles %-change: -2.11% -1.57%
Cycles are helped.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4142>

src/compiler/glsl/float64.glsl

@@ -537,9 +537,7 @@ __roundAndPackInt64(uint zSign, uint zFrac0, uint zFrac1, uint zFrac2)
 int
 __countLeadingZeros32(uint a)
 {
-   int shiftCount;
-   shiftCount = mix(31 - findMSB(a), 32, a == 0u);
-   return shiftCount;
+   return 31 - findMSB(a);
 }
 
 /* Takes an abstract floating-point value having sign `zSign', exponent `zExp',