[SimplifyCFG] isSafeToSpeculateStore now ignores debug info

This patch fixes PR27615.

@llvm.dbg.value instructions no longer count towards the maximum number of
instructions to look back at in the instruction list when searching for a
store instruction. This should make the output consistent between debug and
non-debug build.

Patch by Henric Karlsson <henric.karlsson@ericsson.com>!

Differential Revision: http://reviews.llvm.org/D19912

llvm-svn: 268512