Reorder struct for better Neon codegen.

I unexpectedly discovered that we can reduce our splat-copy ops
by one instruction by swapping these struct fields. This is
apparently because we can wedge a right-shift by 32 into an add
instruction. (`uxtw` means zero-extend.)

Before (splat_2_constants):
    bcf4: 28 04 40 f9  	ldr	x8, [x1, #8]
    bcf8: 09 fd 60 d3  	lsr	x9, x8, #32          <--- eliminated
    bcfc: 30 01 27 1e  	fmov	s16, w9
    bd00: 10 06 04 4e  	dup.4s	v16, v16[0]
    bd04: 88 40 28 8b  	add	x8, x4, w8, uxtw     <--- changed
    bd08: 10 41 00 ad  	stp	q16, q16, [x8]
    bd0c: 25 0c 41 f8  	ldr	x5, [x1, #16]!
    bd10: a0 00 1f d6  	br	x5

After:
    baa0: 28 04 40 f9  	ldr	x8, [x1, #8]
    baa4: 10 01 27 1e  	fmov	s16, w8
    baa8: 10 06 04 4e  	dup.4s	v16, v16[0]
    baac: 88 80 48 8b  	add	x8, x4, x8, lsr #32
    bab0: 10 41 00 ad  	stp	q16, q16, [x8]
    bab4: 25 0c 41 f8  	ldr	x5, [x1, #16]!
    bab8: a0 00 1f d6  	br	x5

(This also saves an op on Haswell!)

Change-Id: Icea7196b42bc4057d697bbf049d368193d46f27e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/679719
Commit-Queue: John Stiles <[email protected]>
Auto-Submit: John Stiles <[email protected]>
Reviewed-by: Brian Osman <[email protected]>
Commit-Queue: Brian Osman <[email protected]>

src/core/SkRasterPipelineOpContexts.h

@@ -156,8 +156,8 @@ struct SkRasterPipeline_TablesCtx {
 using SkRPOffset = uint32_t;
 
 struct SkRasterPipeline_ConstantCtx {
-    SkRPOffset dst;
     float value;
+    SkRPOffset dst;
 };
 
 struct SkRasterPipeline_UniformCtx {