Subzero: Fix a register allocation issue for "advanced phi lowering".

tests_lit/llvm2ice_tests/phi.ll

 ; This tests some of the subtleties of Phi lowering.  In particular,
 ; it tests that it does the right thing when it tries to enable
 ; compare/branch fusing.
 
 ; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 --phi-edge-split=0 \
 ; RUN:   | FileCheck %s
+; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 --phi-edge-split=1 \
+; RUN:   | FileCheck --check-prefix=SPLIT %s
 
 define internal i32 @testPhi1(i32 %arg) {
 entry:
   %cmp1 = icmp sgt i32 %arg, 0
   br i1 %cmp1, label %next, label %target
 next:
   br label %target
 target:
   %merge = phi i1 [ %cmp1, %entry ], [ false, %next ]
   %result = zext i1 %merge to i32
@@ skipping 75 matching lines @@
 
 ; CHECK-LABEL: testPhi3
 ; CHECK: push [[EBX:.*]]
 ; CHECK: mov {{.*}},DWORD PTR [esp
 ; CHECK: mov
 ; CHECK: mov {{.*}},DWORD PTR [[ADDR:.*0x3e8]]
 ; CHECK: cmp {{.*}},0x0
 ; CHECK: jne
 ; CHECK: mov DWORD PTR [[ADDR]]
 ; CHECK: pop [[EBX]]
+
+; Test of "advanced phi lowering" with undef phi arg (integer vector).
+define internal <4 x i32> @test_split_undef_int_vec(<4 x i32> %arg, i32 %cond) {
+entry:
+  %cmp = icmp eq i32 %cond, 0
+  br i1 %cmp, label %eq, label %exit
+eq:
+  br label %exit
+exit:
+  %merge = phi <4 x i32> [ %arg, %entry ], [ undef, %eq ]
+  ret <4 x i32> %merge
+}
+; SPLIT-LABEL: test_split_undef_int_vec
+; SPLIT: pxor
+
+; Test of "advanced phi lowering" with undef phi arg (float vector).
+define internal <4 x float> @test_split_undef_float_vec(<4 x float> %arg, i32 %cond) {
+entry:
+  %cmp = icmp eq i32 %cond, 0
+  br i1 %cmp, label %eq, label %exit
+eq:
+  br label %exit
+exit:
+  %merge = phi <4 x float> [ %arg, %entry ], [ undef, %eq ]
+  ret <4 x float> %merge
+}
+; SPLIT-LABEL: test_split_undef_float_vec
+; SPLIT: pxor
+
+; Test of "advanced phi lowering" with undef phi arg (integer scalar).
+define internal i32 @test_split_undef_int_scalar(i32 %arg, i32 %cond) {
+entry:
+  %cmp = icmp eq i32 %cond, 0
+  br i1 %cmp, label %eq, label %exit
+eq:
+  br label %exit
+exit:
+  %merge = phi i32 [ %arg, %entry ], [ undef, %eq ]
+  ret i32 %merge
+}
+; SPLIT-LABEL: test_split_undef_int_scalar
+; SPLIT: mov {{.*}},0x0
+
+; Test of "advanced phi lowering" with undef phi arg (float scalar).
+define internal float @test_split_undef_float_scalar(float %arg, i32 %cond) {
+entry:
+  %cmp = icmp eq i32 %cond, 0
+  br i1 %cmp, label %eq, label %exit
+eq:
+  br label %exit
+exit:
+  %merge = phi float [ %arg, %entry ], [ undef, %eq ]
+  ret float %merge
+}
+; SPLIT-LABEL: test_split_undef_float_scalar
+; SPLIT: movss {{.*}},DWORD PTR