cmd/compile: make alg gen prologue/epilogue match

With this, the start and end of geneq and genhash
are parallel. This removes a few rare nilchecks
from generated hash functions, but nothing
to write home about.

Change-Id: I3b4836111d04daa6f6834a579bbec374a3f42c70
Reviewed-on: https://go-review.googlesource.com/20456
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/compile/internal/gc/alg.go

@@ -289,7 +289,9 @@ func genhash(sym *Sym, t *Type) {
 	old_safemode := safemode
 
 	safemode = 0
+	Disable_checknil++
 	funccompile(fn)
+	Disable_checknil--
 	safemode = old_safemode
 }
 
@@ -357,6 +359,7 @@ func geneq(sym *Sym, t *Type) {
 	tfn.Rlist.Append(n)
 
 	funchdr(fn)
+	typecheck(&fn.Func.Nname.Name.Param.Ntype, Etype)
 
 	// geneq is only called for types that have equality but
 	// cannot be handled by the standard algorithms,