cmd/compile: fold LEAQ/ADDQconst into SETx ops

This saves an instruction and a register. The new rules
match ~4900 times during all.bash.

Change-Id: I2f867c5e70262004e31f545f3bb89e939c45b718
Reviewed-on: https://go-review.googlesource.com/94767Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1143,7 +1143,7 @@ var linuxAMD64Tests = []*asmTest{
 			return x > 4
 		}
 		`,
-		pos: []string{"\tSETHI\t\\("},
+		pos: []string{"\tSETHI\t.*\\(SP\\)"},
 	},
 	// Check that len() and cap() div by a constant power of two
 	// are compiled into SHRQ.

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -1074,6 +1074,8 @@
     (MOV(Q|L|W|B|SS|SD|O)load  [off1+off2] {sym} ptr mem)
 (MOV(Q|L|W|B|SS|SD|O)store  [off1] {sym} (ADDQconst [off2] ptr) val mem) && is32Bit(off1+off2) ->
 	(MOV(Q|L|W|B|SS|SD|O)store  [off1+off2] {sym} ptr val mem)
+(SET(L|G|B|A|LE|GE|BE|AE|EQ|NE)mem [off1] {sym} (ADDQconst [off2] base) val mem) && is32Bit(off1+off2) ->
+	(SET(L|G|B|A|LE|GE|BE|AE|EQ|NE)mem [off1+off2] {sym} base val mem)
 
 // Fold constants into stores.
 (MOVQstore [off] {sym} ptr (MOVQconst [c]) mem) && validValAndOff(c,off) ->
@@ -1099,6 +1101,9 @@
 	(MOV(Q|L|W|B|SS|SD|O)store [off1+off2] {mergeSym(sym1,sym2)} base val mem)
 (MOV(Q|L|W|B)storeconst [sc] {sym1} (LEAQ [off] {sym2} ptr) mem) && canMergeSym(sym1, sym2) && ValAndOff(sc).canAdd(off) ->
 	(MOV(Q|L|W|B)storeconst [ValAndOff(sc).add(off)] {mergeSym(sym1, sym2)} ptr mem)
+(SET(L|G|B|A|LE|GE|BE|AE|EQ|NE)mem [off1] {sym1} (LEAQ [off2] {sym2} base) val mem)
+	&& is32Bit(off1+off2) && canMergeSym(sym1, sym2) ->
+	(SET(L|G|B|A|LE|GE|BE|AE|EQ|NE)mem [off1+off2] {mergeSym(sym1,sym2)} base val mem)
 
 // generating indexed loads and stores
 (MOV(B|W|L|Q|SS|SD)load [off1] {sym1} (LEAQ1 [off2] {sym2} ptr idx) mem) && is32Bit(off1+off2) && canMergeSym(sym1, sym2) ->