Make CanFallThrough more intelligent (so it can handle blocks with (e.g.) no

successors), and make island block movement more general.

This compiles CodeGen/X86/2006-04-27-ISelFoldingBug.ll to:

_loadAndRLEsource_no_exit_2E_1_label_2E_0:
        subl $8, %esp
        movl %esi, 4(%esp)
        movl %ebx, (%esp)
        movl 16(%esp), %eax
        movl 12(%esp), %ecx
LBB1_3: #label.0
        movl _last, %edx
        movl %edx, %esi
        incl %esi
        movl %esi, _last
        movl %ecx, %ebx
        # TRUNCATE movb %bl, %bl
        movl _block, %esi
        movb %bl, 1(%esi,%edx)
        cmpl %eax, _last
        jge LBB1_2      #codeRepl5.exitStub
LBB1_4: #label.0
        cmpl $257, %ecx
        je LBB1_2       #codeRepl5.exitStub
LBB1_1: #label.0.no_exit.1_crit_edge.exitStub
        movl $1, %eax
        movl (%esp), %ebx
        movl 4(%esp), %esi
        addl $8, %esp
        ret
LBB1_2: #codeRepl5.exitStub
        xorl %eax, %eax
        movl (%esp), %ebx
        movl 4(%esp), %esi
        addl $8, %esp
        ret

instead of:

_loadAndRLEsource_no_exit_2E_1_label_2E_0:
        subl $8, %esp
        movl %esi, 4(%esp)
        movl %ebx, (%esp)
        movl 16(%esp), %eax
        movl 12(%esp), %ecx
        jmp LBB1_3      #label.0
LBB1_1: #label.0.no_exit.1_crit_edge.exitStub
        movl $1, %eax
        movl (%esp), %ebx
        movl 4(%esp), %esi
        addl $8, %esp
        ret
LBB1_2: #codeRepl5.exitStub
        xorl %eax, %eax
        movl (%esp), %ebx
        movl 4(%esp), %esi
        addl $8, %esp
        ret
LBB1_3: #label.0
        movl _last, %edx
        movl %edx, %esi
        incl %esi
        movl %esi, _last
        movl %ecx, %ebx
        # TRUNCATE movb %bl, %bl
        movl _block, %esi
        movb %bl, 1(%esi,%edx)
        cmpl %eax, _last
        jge LBB1_2      #codeRepl5.exitStub
LBB1_4: #label.0
        cmpl $257, %ecx
        jne LBB1_1      #label.0.no_exit.1_crit_edge.exitStub
        jmp LBB1_2      #codeRepl5.exitStub

... which is much better layout :)

llvm-svn: 31282