[sancov] Generalize the code to get the previous instruction to multiple architectures
sancov subtracts one from the address to get the previous instruction,
which makes sense on x86_64, but not on other platforms.
This change ensures that the offset is correct for different platforms.
The logic for computing the offset is copied from sanitizer_common.
Differential Revision: https://reviews.llvm.org/D53039
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344103
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