MIR Tests: Make the basic block successor test an X86 specific test.

MIR Tests: Make the basic block successor test an X86 specific test.

This commit moves and transforms the generic test
'CodeGen/MIR/successor-basic-blocks.mir' into an X86 specific test
'CodeGen/MIR/X86/successor-basic-blocks.mir'. This change is required in order
to enable the machine verifier for the MIR parser, as the machine verifier
verifies that the machine basic blocks contain instructions that actually
determine the machine basic block successors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243123 91177308-0d34-0410-b5e6-96231b3b80d8