Enable FDiv(x, x) = 1.0 for float redundant division

Uncomment the FDiv x/x rule that was disabled due to NaN/zero concerns.
The Rust optimizer already assumes fast-math for all float rules (FSub(x,x)=0
fires unconditionally), making this consistent with existing behavior.

Combined with the FNeg distribution rules (FDiv(FNeg(a),b) → FNeg(FDiv(a,b))
and FDiv(a,FNeg(b)) → FNeg(FDiv(a,b))), this also handles -x/x → -1.0 and
x/(-x) → -1.0, matching the C++ FoldRedundantDiv behavior.

Author: LegNeato

rust/spirv-tools-opt/src/rules/floating_point.egg

@@ -111,9 +111,10 @@
 (rule ((= e (FDiv x (FConst c))) (= c -1.0))
       ((union e (FNeg x))))
 
-; x / x = 1 (assuming not zero or NaN)
-; Note: commented out due to NaN/zero concerns
-; (rewrite (FDiv x x) (FConst 1.0))
+; x / x = 1.0 (fast-math: assumes x is not zero or NaN) - ONE-DIRECTIONAL
+; Combined with FNeg distribution rules below, also handles -x/x and x/(-x)
+(rule ((= e (FDiv x x)))
+      ((union e (FConst 1.0))))
 
 ; x - x = 0 - ONE-DIRECTIONAL
 (rule ((= e (FSub x x)))