[dev.simd] simd: clean up mask load comments

Addressing a comment from CL 710915.

Change-Id: Id65f525130b5b626ea7017aebc93a4b3b0c93d84
Reviewed-on: https://go-review.googlesource.com/c/go/+/711780
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

Author: JunyangShao

src/simd/_gen/simdgen/gen_simdTypes.go

@@ -182,13 +182,17 @@ func (x {{.Name}}) Store(y *[{{.Lanes}}]{{.Base}})
 
 const simdMaskFromValTemplate = `
 // {{.Name}}FromBits constructs a {{.Name}} from a bitmap value, where 1 means set for the indexed element, 0 means unset.
+{{- if ne .Lanes .LanesContainer}}
 // Only the lower {{.Lanes}} bits of y are used.
+{{- end}}
 //
 // Asm: KMOV{{.IntelSizeSuffix}}, CPU Feature: AVX512
 func {{.Name}}FromBits(y uint{{.LanesContainer}}) {{.Name}}
 
 // ToBits constructs a bitmap from a {{.Name}}, where 1 means set for the indexed element, 0 means unset.
+{{- if ne .Lanes .LanesContainer}}
 // Only the lower {{.Lanes}} bits of y are used.
+{{- end}}
 //
 // Asm: KMOV{{.IntelSizeSuffix}}, CPU Features: AVX512
 func (x {{.Name}}) ToBits() uint{{.LanesContainer}}