[PowerPC] Make verifier happy when lowering `llvm.trap` (#77266)

`llvm.trap` is lowered to `PPC::TRAP` and `PPC::TRAP` is set as
terminator. Verifier complains about terminator should not lie in the
middle of an MBB. See #77095.

Fix it by removing `isTerminator` and `isBarrier` and then set `isTrap`
which was introduced by https://reviews.llvm.org/D48836# and is being
used by X86 and AArch64.

`PPC::TRAP` is not a hardware memory barrier and `llvm.trap` doesn't
indicate a memory barrier either.