Fix operator% for f32 and vecN<f32>

https://github.com/gpuweb/gpuweb/pull/1945 changes the SPIR-V mapping of this operator so that it now maps to OpFRem instead of OpFMod. Polyfill OpFMod with `x - y * floor(x / y)`

Also map the MSL output of this operator to use `fmod()`.

Behavior of this operator is now consistent across all backends.

Fixed: tint:945
Fixed: tint:977
Fixed: tint:1010
Change-Id: Iefa009b905989c55ace24e073ab0e261c7cf69b0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58393
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>

test/expressions/binary/mod/scalar-scalar/f32.wgsl.expected.msl

@@ -1,19 +1,10 @@
-SKIP: FAILED
-
 #include <metal_stdlib>
 
 using namespace metal;
 kernel void f() {
   float const a = 1.0f;
   float const b = 2.0f;
-  float const r = (a % b);
+  float const r = fmod(a, b);
   return;
 }
 
-Compilation failed: 
-
-program_source:7:22: error: invalid operands to binary expression ('const float' and 'const float')
-  float const r = (a % b);
-                   ~ ^ ~
-
-

test/expressions/binary/mod/scalar-scalar/f32.wgsl.expected.spvasm

@@ -15,6 +15,6 @@
     %float_2 = OpConstant %float 2
           %f = OpFunction %void None %1
           %4 = OpLabel
-          %8 = OpFMod %float %float_1 %float_2
+          %8 = OpFRem %float %float_1 %float_2
                OpReturn
                OpFunctionEnd