LoopVectorizer: Fix 15830. When scalarizing and unrolling stores make
sure that the order in which the elements are scalarized is the same
as the original order.
This fixes a miscompilation in FreeBSD's regex library.
This should fix lib/libc/regex/regcomp.c at -O3 with clang 3.3 r178860
on CPUs with SSE. Before this change, the vectorizer could incorrectly
rearrange the second loop in computejumps(), leading to possibly invalid
entries in the re_gets::charjump table.
The net result was that for example "sed s/@CC@/foo/" failed to work
correctly, leading to trouble with many configure scripts.
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