Enforce stricter liveness rules for PHIs.

Verify that all paths from the entry block to a virtual register read
pass through a def. Enable this check even when MRI->isSSA() is false.

Verify that the live range of a virtual register is live out of all
predecessor blocks, even for PHI-values.

This requires that PHIElimination sometimes inserts IMPLICIT_DEF
instruction in predecessor blocks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159150 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/Generic/undef-phi.ll

@@ -0,0 +1,26 @@
+; RUN: llc < %s -verify-machineinstrs -verify-coalescing
+;
+; This function has a PHI with one undefined input. Verify that PHIElimination
+; inserts an IMPLICIT_DEF instruction in the predecessor so all paths to the use
+; pass through a def.
+
+%struct.xx_stack = type { i32, %struct.xx_stack* }
+
+define i32 @push(%struct.xx_stack* %stack) nounwind uwtable readonly ssp {
+entry:
+  %tobool1 = icmp eq %struct.xx_stack* %stack, null
+  br i1 %tobool1, label %for.end, label %for.body
+
+for.body:
+  %stack.addr.02 = phi %struct.xx_stack* [ %0, %for.body ], [ %stack, %entry ]
+  %next = getelementptr inbounds %struct.xx_stack* %stack.addr.02, i64 0, i32 1
+  %0 = load %struct.xx_stack** %next, align 8
+  %tobool = icmp eq %struct.xx_stack* %0, null
+  br i1 %tobool, label %for.end, label %for.body
+
+for.end:
+  %top.0.lcssa = phi %struct.xx_stack* [ undef, %entry ], [ %stack.addr.02, %for.body ]
+  %first = getelementptr inbounds %struct.xx_stack* %top.0.lcssa, i64 0, i32 0
+  %1 = load i32* %first, align 4
+  ret i32 %1
+}