InlineSpiller: Delete assert that implicit_def has no implicit operands (#69087)

It's not a verifier enforced property that implicit_def may only have
one operand. Fixes assertions after the coalescer implicit-defs to
preserve super register liveness to arbitrary instructions.

For some reason I'm unable to reproduce this as a MIR test running only
the allocator for the x86 test. Not sure it's worth keeping around.