[AArch64] [Windows] SEH opcodes should be scheduling boundaries.

Prevents the post-RA scheduler from modifying the prologue sequences
emitting by frame lowering. This is roughly similar to what we do for
other targets: TargetInstrInfo::isSchedulingBoundary checks
isPosition(), which checks for CFI_INSTRUCTION.

isSEHInstruction is taken from D50288; it'll land with whatever patch
lands first.

Differential Revision: https://reviews.llvm.org/D53851

llvm-svn: 345634