soft-fp64/fadd: Delete a redundant condition check

Previous condition checks already guaranteen that expDiff != 0 and
!(expDiff > 0), so expDiff < 0 is the only option left.

The main purpose of this commit is to prepare for "soft-fp64/fadd: Move
common code out of both branches of an if-statement".

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: 815491 -> 812590 (-0.36%)
instructions in affected programs: 753668 -> 750767 (-0.38%)
helped: 74
HURT: 0
helped stats (abs) min: 3 max: 281 x̄: 39.20 x̃: 25
helped stats (rel) min: 0.29% max: 0.73% x̄: 0.42% x̃: 0.40%
95% mean confidence interval for instructions value: -48.50 -29.91
95% mean confidence interval for instructions %-change: -0.45% -0.40%
Instructions are helped.

total cycles in shared programs: 6813681 -> 6798039 (-0.23%)
cycles in affected programs: 6381699 -> 6366057 (-0.25%)
helped: 74
HURT: 0
helped stats (abs) min: 24 max: 1488 x̄: 211.38 x̃: 149
helped stats (rel) min: 0.20% max: 0.44% x̄: 0.26% x̃: 0.25%
95% mean confidence interval for cycles value: -261.68 -161.08
95% mean confidence interval for cycles %-change: -0.28% -0.25%
Cycles are helped.

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

src/compiler/glsl/float64.glsl

@@ -708,7 +708,7 @@ __fadd64(uint64_t a, uint64_t b)
          __shift64ExtraRightJamming(
             bFracHi, bFracLo, 0u, expDiff, bFracHi, bFracLo, zFrac2);
          zExp = aExp;
-      } else if (expDiff < 0) {
+      } else {
          if (bExp == 0x7FF) {
             bool propagate = (bFracHi | bFracLo) != 0u;
             return mix(__packFloat64(aSign, 0x7ff, 0u, 0u), __propagateFloat64NaN(a, b), propagate);