Optimize _ pattern in syntax-case clauses (#728)

Avoid emitting a call to $syntax-dispatch for _ patterns and emit ()
instead. This allow cp0 optimizations for such patterns.

Author: Jarhmander

LOG

@@ -2455,3 +2455,5 @@
     boot/*/scheme.h c/externs.h c/number.c c/scheme.c csug/foreign.stex
     mats/foreign1.c mats/foreign.ms release_notes/release_notes.stex
     s/mkheader.ss
+- optimize simple _ patterns in syntax-case
+    s/syntax.ss

s/syntax.ss

@@ -6423,12 +6423,16 @@
            (let ([y (gen-var 'tmp)])
              (build-let no-source
                (list y)
-               (list (if (eq? p 'any)
-                         (build-primcall no-source 3 'list
-                           (build-lexical-reference no-source x))
-                         (build-primcall no-source 3 '$syntax-dispatch
-                           (build-lexical-reference no-source x)
-                           (build-data no-source p))))
+               (list (cond
+                       [(eq? p 'any)
+                        (build-primcall no-source 3 'list
+                                        (build-lexical-reference no-source x))]
+                       [(eq? p '_)
+                        (build-data no-source '())]
+                       [else
+                        (build-primcall no-source 3 '$syntax-dispatch
+                                        (build-lexical-reference no-source x)
+                                        (build-data no-source p))]))
                (let-syntax ([y (identifier-syntax
                                  (build-lexical-reference no-source y))])
                  (build-conditional no-source