Revert r304824 "Fix PR23384 (part 3 of 3)"

This seems to be interacting badly with ASan somehow, causing false reports of
heap-buffer overflows: PR33514.

> Summary:
> The patch makes instruction count the highest priority for
> LSR solution for X86 (previously registers had highest priority).
>
> Reviewers: qcolombet
>
> Differential Revision: http://reviews.llvm.org/D30562
>
> From: Evgeny Stupachenko <evstupac@gmail.com>

llvm-svn: 305720