Expand shenandoah write barrier as C2 IR

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/shenandoah/wb2ir/webrev.00/

This expands the write barrier to c2 IR after most optimizations are
over. It also takes care of finding a dominating null check and reshapes
the graph to enable implicit null checks. This method:

    static void m9(A a) {
        a.f = 0x42;
    }

is compiled to (on x86):

  0x00007fa4bcb4bb4c: movzbl 0x640(%r15),%r11d
  0x00007fa4bcb4bb54: test   %r11d,%r11d
  0x00007fa4bcb4bb57: jne    0x00007fa4bcb4bb70
 ;; B2: #	B6 B3 <- B1  Freq: 0.999

  0x00007fa4bcb4bb59: mov    -0x8(%rsi),%rax    ; implicit exception: dispatches to 0x00007fa4bcb4bb83
 ;; B3: #	N1 <- B2 B5  Freq: 0.999999

  0x00007fa4bcb4bb5d: movl   $0x42,0x10(%rax)   ;*putfield f {reexecute=0 rethrow=0 return_oop=0}
                                                ; - TestShenandoahBarrier::m9 at 3 (line 66)

  0x00007fa4bcb4bb64: add    $0x10,%rsp
  0x00007fa4bcb4bb68: pop    %rbp
  0x00007fa4bcb4bb69: test   %eax,0x14487491(%rip)        # 0x00007fa4d0fd3000
                                                ;   {poll_return}
  0x00007fa4bcb4bb6f: retq   
 ;; B4: #	B6 B5 <- B1  Freq: 0.000999987

  0x00007fa4bcb4bb70: mov    -0x8(%rsi),%rdi    ; implicit exception: dispatches to 0x00007fa4bcb4bb83
 ;; B5: #	B3 <- B4  Freq: 0.000999986

  0x00007fa4bcb4bb74: movabs $0x7fa4bc94a8e4,%r10
  0x00007fa4bcb4bb7e: callq  *%r10
  0x00007fa4bcb4bb81: jmp    0x00007fa4bcb4bb5d

and on aarch64:

  0x000003ff84b49cd4: ldrb      w11, [xthread,#1600]
  0x000003ff84b49cd8: dmb       ishld
  0x000003ff84b49cdc: cbnz      w11, 0x000003ff84b49d00
 ;; B2: #       B6 B3 <- B1  Freq: 0.999

  0x000003ff84b49ce0: ldr       x0, [x1,#-8]    ; implicit exception: dispatches to 0x000003ff84b49d0c
 ;; B3: #       N1 <- B2 B5  Freq: 0.999999

 ;; 0x42
  0x000003ff84b49ce4: mov       w10, #0x42                      // #66
  0x000003ff84b49ce8: str       w10, [x0,#16]   ;*synchronization entry
                                                ; - TestShenandoahBarrier::m9 at -1 (line 66)

  0x000003ff84b49cec: ldp       xfp, xlr, [sp,#16]
  0x000003ff84b49cf0: add       sp, sp, #0x20
  0x000003ff84b49cf4: adrp      xscratch1, 0x000003ff976d0000
                                                ;   {poll_return}
  0x000003ff84b49cf8: ldr       wzr, [xscratch1]  ;   {poll_return}
  0x000003ff84b49cfc: ret
 ;; B4: #       B6 B5 <- B1  Freq: 0.000999987

  0x000003ff84b49d00: ldr       x0, [x1,#-8]    ; implicit exception: dispatches to 0x000003ff84b49d0c
 ;; B5: #       B3 <- B4  Freq: 0.000999986

  0x000003ff84b49d04: bl        Stub::shenandoah_wb+4 0x000003ff849bf994
                                                ;   {runtime_call StubRoutines (3)}
  0x000003ff84b49d08: b 0x000003ff84b49ce4

This is off by default for now as I'm seeing some hangs on aarch64.

Roland.