Subzero: Use -filetype instead of -ias and -elf-writer.

This matches LLVM's -filetype={obj,asm} options, and also adds -filetype=iasm to get textual rendering with the integrated assembler.

It also removes the old illegal combination of -ias=0 -elf-writer.

BUG= none
R=jvoung@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/920953002

src/IceDefs.h

diff --git a/src/IceDefs.h b/src/IceDefs.h
index 19b0694..1ee3d23 100644
--- a/src/IceDefs.h
+++ b/src/IceDefs.h
@@ -177,6 +177,12 @@
 };
 typedef uint32_t VerboseMask;
 
+enum FileType {
+  FT_Elf, // ELF .o file
+  FT_Asm, // Assembly .s file
+  FT_Iasm // "Integrated assembler" .byte-style .s file
+};
+
 typedef llvm::raw_ostream Ostream;
 typedef llvm::raw_fd_ostream Fdstream;