* decode.scm: Tweak various comments.

	* decode.scm: Tweak various comments.
	(/opcode-slots): Add FIXME.
	(/build-decode-table-guts): Add assert.
	* utils-sim.scm (/gen-set-itype-and-extract): New function.
	(/gen-bracketed-set-itype-and-extract): New function.
	(/gen-decode-default-entry): Rewrite.
	(/table-guts-to-mask, /all-opcode-bits-used?): New functions.
	(/gen-decode-insn-entry): New arg table-guts-thus-far, all callers
	updated.  Don't unnecessarily emit check for whether all opcode bits
	have been examined.
	(/gen-decode-expr-set-itype): Delete.
	(/gen-decode-expr-entry): Update.
	(/gen-decode-table-entry): New arg table-guts-thus-far, all callers
	updated.  Keep track of decoder tables used thus far.
	(/gen-decoder-switch): Ditto.
	* utils.scm (word-bit-value): New function.