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7c: dont replace SXTW instruction following MOVW $const, r

All the MOVW instructions sign extend to full
register size, but "MOVW $cosnt, R" is the exception,
being equivalent to "MOVWU $const, R" (for historical
reasons?).

This fixes hjfs regression.

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

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

@@ -252,11 +252,14 @@

 			if((r1 = findset(r, &p->from)) != R){

 				p1 = r1->prog;

 				switch(p1->as){

+				case AMOVW:

+					/* MOVW $const,r; does not sign extend */

+					if(p1->from.type == D_CONST && (p1->from.offset & 0x80000000) != 0)

+						break;

 				case AMOVB:

-				case AMOVBU:

 				case AMOVH:

+				case AMOVBU:

 				case AMOVHU:

-				case AMOVW:

 					if(p1->to.type == p->from.type && p1->to.reg == p->from.reg)

 						p->as = AMOVW;

 					break;