[LoopUnswitch] Properly update MSSA if header has non-clobbering stores.

This patch fixes updating MemorySSA if the header contains memory
defs that do not clobber a duplicated instruction. We need to find the
first defining access outside the loop body and use that as defining
access of the duplicated instruction.

This fixes a crash caused by bee48685.

(Cherry-picked on the 12.x release branch from
10c57268)