cmd/internal/obj/loong64: clean up code for short conditional branches

Untangle the logic so the preparation of operands and actual assembling
(branch range checking included) are properly separated, making future
changes easier to review and maintain. No functional change intended.

Change-Id: I1f73282f9d92ff23d84846453d3597ba66d207d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/478376
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: abner chenc <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
(cherry picked from commit 1ae306a)

Author: xen0n

src/cmd/internal/obj/loong64/asm.go

@@ -1179,26 +1179,26 @@ func (c *ctxt0) asmout(p *obj.Prog, o *Optab, out []uint32) {
 
 	case 6: // beq r1,[r2],sbra
 		v := int32(0)
-		vcmp := int32(0)
 		if p.To.Target() != nil {
 			v = int32(p.To.Target().Pc-p.Pc) >> 2
 		}
-		if v < 0 {
-			vcmp = -v
-		}
-		if (p.As == ABFPT || p.As == ABFPF) && ((uint32(vcmp))>>21)&0x7FF != 0 {
-			c.ctxt.Diag("21 bit-width, short branch too far\n%v", p)
-		} else if p.As != ABFPT && p.As != ABFPF && (v<<16)>>16 != v {
-			c.ctxt.Diag("16 bit-width, short branch too far\n%v", p)
-		}
+		rd, rj := p.Reg, p.From.Reg
 		if p.As == ABGTZ || p.As == ABLEZ {
-			o1 = OP_16IRR(c.opirr(p.As), uint32(v), uint32(p.Reg), uint32(p.From.Reg))
-		} else if p.As == ABFPT || p.As == ABFPF {
-			// BCNEZ cj offset21 ,cj = fcc0
-			// BCEQZ cj offset21 ,cj = fcc0
+			rd, rj = rj, rd
+		}
+		switch p.As {
+		case ABFPT, ABFPF:
+			if (v<<11)>>11 != v {
+				c.ctxt.Diag("21 bit-width, short branch too far\n%v", p)
+			}
+			// FCC0 is the implicit source operand, now that we
+			// don't register-allocate from the FCC bank.
 			o1 = OP_16IR_5I(c.opirr(p.As), uint32(v), uint32(REG_FCC0))
-		} else {
-			o1 = OP_16IRR(c.opirr(p.As), uint32(v), uint32(p.From.Reg), uint32(p.Reg))
+		default:
+			if (v<<16)>>16 != v {
+				c.ctxt.Diag("16 bit-width, short branch too far\n%v", p)
+			}
+			o1 = OP_16IRR(c.opirr(p.As), uint32(v), uint32(rj), uint32(rd))
 		}
 
 	case 7: // mov r, soreg