[X86] Properly chain PROBED_ALLOCA / SEG_ALLOCA (#116508)
These nodes should appear between CALLSEQ_START / CALLSEQ_END.
Previously, they could be scheduled after CALLSEQ_END because the nodes
didn't update the chain.
The change in a test is due to X86 call frame optimizer pass bailing out
for a particular call when CALLSEQ_START / CALLSEQ_END are not in the
same basic block. This happens because SEG_ALLOCA is expanded into a
sequence of basic blocks early. It didn't bail out before because the
closing CALLSEQ_END was scheduled before SEG_ALLOCA, in the same basic
block as CALLSEQ_START.
While here, simplify creation of these nodes: allocating a virtual
register and copying `Size` into it were unnecessary.
