build: more "undefined behavior" fixes

Fixes #5764.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13441051

Author: rsc

src/cmd/6g/gsubr.c

@@ -540,7 +540,7 @@ ginscon(int as, vlong c, Node *n2)
 
 	nodconst(&n1, types[TINT64], c);
 
-	if(as != AMOVQ && (c < -1LL<<31 || c >= 1LL<<31)) {
+	if(as != AMOVQ && (c < -(1LL<<31) || c >= 1LL<<31)) {
 		// cannot have 64-bit immediokate in ADD, etc.
 		// instead, MOV into register first.
 		regalloc(&ntmp, types[TINT64], N);

src/cmd/6l/obj.c

@@ -346,7 +346,7 @@ zaddr(char *pn, Biobuf *f, Adr *a, Sym *h[])
 		a->offset = BGETLE4(f);
 		if(t & T_64) {
 			a->offset &= 0xFFFFFFFFULL;
-			a->offset |= (vlong)BGETLE4(f) << 32;
+			a->offset |= (uvlong)BGETLE4(f) << 32;
 		}
 	}
 	a->sym = S;

src/cmd/6l/span.c

@@ -1237,6 +1237,8 @@ doasm(Prog *p)
 		break;
 	}
 
+	if(z >= nelem(o->op))
+		sysfatal("asmins bad table %P", p);
 	op = o->op[z];
 	if(op == 0x0f) {
 		*andptr++ = op;

src/libbio/bgetc.c

@@ -83,7 +83,7 @@ Bgetle4(Biobuf *bp)
 
 	l = Bgetle2(bp);
 	h = Bgetle2(bp);
-	return l|(h<<16);
+	return l|((uint32)h<<16);
 }
 
 int