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7c: fix wrong type on OASxxx operations

the bug can be reproduced with the following test case:

#include <u.h>
#include <libc.h>

void
main()
{
	int size = 1;
	size*=1.5;
	exits(0);
}

this produces the following assembly:

	TEXT	main+0(SB),0,$16
	MOVW	$1,R1
	FCVTZSDW	$1.50000000000000000e+00,R2	<- tries to convert rhs to int??
	MULW	R2,R1,R2 <- multiplication done in int? bug!
	MOV	$0,R0
	BL	,exits+0(SB)
	RETURN	,
	END	,

the confusion comes from the *= operation using the wrong type
for the multiplication. in this case we should use the float
type of the rhs, do the operation, and then convert the result
back to int type of the lhs.

this change ports the same logic from 5c's getasop().

--- a/sys/src/cmd/7c/cgen.c

+++ b/sys/src/cmd/7c/cgen.c

@@ -287,28 +287,25 @@

 				reglcgen(&nod2, l, Z);

 			else

 				nod2 = *l;

-			regalloc(&nod, n, nn);

-			cgen(r, &nod);

+			regalloc(&nod1, r, Z);

+			cgen(r, &nod1);

 		} else {

-			regalloc(&nod, n, nn);

-			cgen(r, &nod);

+			regalloc(&nod1, r, Z);

+			cgen(r, &nod1);

 			if(l->addable < INDEXED)

 				reglcgen(&nod2, l, Z);

 			else

 				nod2 = *l;

 		}

-		regalloc(&nod1, n, Z);

-		gopcode(OAS, &nod2, Z, &nod1);

-		if(nod1.type->etype != nod.type->etype){

-			regalloc(&nod3, &nod, Z);

-			gmove(&nod1, &nod3);

-			regfree(&nod1);

-			nod1 = nod3;

-		}

-		gopcode(o, &nod, &nod1, &nod);

+		if(nod1.type == nod2.type || !typefd[nod1.type->etype])

+			regalloc(&nod, &nod2, nn);

+		else

+			regalloc(&nod, &nod1, Z);

+		gmove(&nod2, &nod);

+		gopcode(o, &nod1, &nod, &nod);

 		gmove(&nod, &nod2);

 		if(nn != Z)

-			gmove(&nod, nn);

+			gmove(&nod2, nn);

 		regfree(&nod);

 		regfree(&nod1);

 		if(l->addable < INDEXED)