[TableGen] Assign created ops to variables and rewrite with PatternRewriter::replaceOp()

Previously we were using PatternRewrite::replaceOpWithNewOp() to both create the new op
inline and rewrite the matched op. That does not work well if we want to generate multiple
ops in a sequence. To support that, this CL changed to assign each newly created op to a
separate variable.

This CL also refactors how PatternEmitter performs the directive dispatch logic.

PiperOrigin-RevId: 233206819