[VM, Compiler] Move compiler to a separate folder.

New folder structure (nested under vm/):

- compiler/
-   jit/         - JIT specific code
-   aot/         - AOT specific code
-   backend/     - all middle-end and back-end code (IL, flow graph)
-   assembler/   - assemblers and disassemblers
-   frontend/    - front ends (AST -> IL, Kernel -> IL)

compiler/README.md would be the documentation root for the compiler
pipeline

Bug: https://github.com/dart-lang/sdk/issues/30575
Change-Id: I2dfd9688793bff737f7632ddc77fca766875ce36
Reviewed-on: https://dart-review.googlesource.com/2940
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>

runtime/vm/code_patcher_ia32.cc

@@ -5,11 +5,11 @@
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_IA32.
 #if defined(TARGET_ARCH_IA32)
 
-#include "vm/assembler.h"
 #include "vm/code_patcher.h"
+#include "vm/compiler/assembler/assembler.h"
+#include "vm/compiler/backend/flow_graph_compiler.h"
 #include "vm/cpu.h"
 #include "vm/dart_entry.h"
-#include "vm/flow_graph_compiler.h"
 #include "vm/instructions.h"
 #include "vm/object.h"
 #include "vm/raw_object.h"