Add phi instruction to Subzero bitcode reader.

tests_lit/reader_tests/phi.ll

+; Test reading phi instructions.
+
+; RUN: llvm-as < %s | pnacl-freeze -allow-local-symbol-tables \
+; RUN:              | %llvm2ice -notranslate -verbose=inst -build-on-read \
+; RUN:                -allow-pnacl-reader-error-recovery \
+; RUN:                -allow-local-symbol-tables \
+; RUN:              | FileCheck %s
+
+; TODO(kschimpf) Add forward reference examples.
+
+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
+  ret i32 %result
+}
+
+; CHECK:      define internal i32 @testPhi1(i32 %arg) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   %cmp1 = icmp sgt i32 %arg, 0
+; CHECK-NEXT:   br i1 %cmp1, label %next, label %target
+; CHECK-NEXT: next:
+; CHECK-NEXT:   br label %target
+; CHECK-NEXT: target:
+; CHECK-NEXT:   %merge = phi i1 [ %cmp1, %entry ], [ false, %next ]
+; CHECK-NEXT:   %result = zext i1 %merge to i32
+; CHECK-NEXT:   ret i32 %result
+; CHECK-NEXT: }
+