[CodeGen] Print bundled instructions using the MIR syntax in -debug output

Old syntax:

BUNDLE implicit-def %r0, implicit-def %r1, implicit %r2
* %r0 = SOME_OP %r2
* %r1 = ANOTHER_OP internal %r0

New syntax:

BUNDLE implicit-def %r0, implicit-def %r1, implicit %r2 {
  %r0 = SOME_OP %r2
  %r1 = ANOTHER_OP internal %r0
}

llvm-svn: 325032