[Hexagon] pX.new cannot be used with p3:0 as producer

[bcain]

Summary

Writes to p3:0 do not produce new values, we should bar any .new
consumer trying to use it as a producer.

[sidneym]

LGTM

llvm/lib/Target/Hexagon/MCTargetDesc/HexagonMCChecker.cpp

 }
 
 // Check legal use of predicate registers.
 bool HexagonMCChecker::checkPredicates() {
   // Check for proper use of new predicate registers.
   for (const auto &I : NewPreds) {
     unsigned P = I;
 
-    if (!Defs.count(P) || LatePreds.count(P)) {
+    if (!Defs.count(P) || LatePreds.count(P) || Defs.count(Hexagon::P3_0)) {
       // Error out if the new predicate register is not defined,
       // or defined "late"
       // (e.g., "{ if (p3.new)... ; p3 = sp1loop0(#r7:2, Rs) }").
       reportErrorNewValue(P);
       return false;
     }
   }
 

llvm/test/MC/Hexagon/c4_newval.s

+# RUN: not llvm-mc -arch=hexagon %s 2>%t; FileCheck --implicit-check-not=error %s <%t
+
+.Lfoo:
+{ p3:0 = r0
+  if (!p0.new) jump:t .Lfoo }
+
+# CHECK: error: register `P0' used with `.new' but not validly modified in the same packet
+
+{ c4 = r0
+  if (!p0.new) jump:t .Lfoo }
+
+# CHECK: error: register `P0' used with `.new' but not validly modified in the same packet
+
+{ c4 = r0
+  p0 = r0
+  if (!p0.new) jump:t .Lfoo }
+
+# CHECK: error: register `P0' used with `.new' but not validly modified in the same packet
+# CHECK: error: register `P3_0' modified more than once

llvm/test/MC/Hexagon/multiple-pc4.s

+# RUN: llvm-mc -arch=hexagon -filetype=asm %s 2>%t; FileCheck --implicit-check-not=error %s <%t
+
+# Check that multiple changes to a predicate in a packet are caught.
+
+  { p0 = cmp.eq (r0, r0); p3:0 = r0 }
+# CHECK: rror: register {{.+}} modified more than once
+
+  { p0 = cmp.eq (r0, r0); c4 = r0 }
+# CHECK: rror: register {{.+}} modified more than once
+
+  p3:0 = r9
+# CHECK-NOT: rror: register {{.+}} modified more than once
+
+# Multiple writes to the same predicate register are permitted:
+
+  { p0 = cmp.eq (r0, r0); p0 = and(p1, p2) }
+# CHECK-NOT: rror: register {{.+}} modified more than once