[CodeGen] Move function splitting tests from X86 to Generic (NFC)

Machine function splitting will become available for AArch64; since MFS
is no longer X86-only, the tests for generic behavior should live
somewhere other than tests/CodeGen/X86.

MFS implementation doesn't vary much across platforms, and most tests
should be identical between X86 and AArch64 besides instruction
selection, so the tests can live together in tests/CodeGen/Generic.

Differential Revision: https://reviews.llvm.org/D157563