cmd/compile: use SEQZ pseudo instruction in RISCV64 boolean rules

This makes the intent clearer, allows for another ellipsis and will aid
in future rewriting. While here, document boolean loads to explain register
contents.

Change-Id: I933db2813826d88819366191fbbea8fcee5e4dda
Reviewed-on: https://go-review.googlesource.com/c/go/+/230120
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/RISCV64Ops.go

@@ -10,6 +10,9 @@ import (
 	"fmt"
 )
 
+// Notes:
+//  - Boolean types occupy the entire register. 0=false, 1=true.
+
 // Suffixes encode the bit width of various instructions:
 //
 // D (double word) = 64 bit int