Add support for undef values in ICE IR.

tests_lit/llvm2ice_tests/undef.ll

+; This test checks that undef values are represented as zero.
+
+; RUN: %llvm2ice --verbose none %s | FileCheck  %s
+; RUN: %llvm2ice -O2 --verbose none %s | FileCheck  %s
+; RUN: %llvm2ice --verbose none %s | FileCheck --check-prefix=ERRORS %s
+; RUN: %llvm2iceinsts %s | %szdiff %s | FileCheck --check-prefix=DUMP %s
+; RUN: %llvm2iceinsts --pnacl %s | %szdiff %s \
+; RUN:                           | FileCheck --check-prefix=DUMP %s
+
+define i32 @undefi32() {
+entry:
+; CHECK-LABEL: undefi32:
+  ret i32 undef
+; CHECK: mov eax, 0
+; CHECK: ret
+}
+
+define i64 @undefi64() {
+entry:
+; CHECK-LABEL: undefi64:
+  ret i64 undef
+; CHECK-DAG: mov eax, 0
+; CHECK-DAG: mov edx, 0
+; CHECK: ret
+}
+
+define float @undeffloat() {
+entry:
+; CHECK-LABEL: undeffloat:
+  ret float undef
+; CHECK-NOT: sub esp
+; CHECK: fld
+; CHECK: ret
+}
+
+; ERRORS-NOT: ICE translation error
+; DUMP-NOT: SZ