Fix the unwinding plan augmentation from x86 assembly

Unwind plan augmentation should compute the plan row at offset x from
the instruction before offset x, but currently we compute it from the
instruction at offset x. Note that this behavior is a regression
introduced when moving the x86 assembly inspection engine to its own
file
(https://github.com/llvm/llvm-project/commit/1c9858b298d79ce82c45a2954096718b39550109#diff-375a2be066db6f34bb9a71442c9b71fcL913);
the original version handled this properly by copying the previous
instruction out before advancing the instruction pointer.

The relevant bug with more info is here: https://bugs.llvm.org/show_bug.cgi?id=43561

Differential Revision: https://reviews.llvm.org/D68454
Patch by Jaroslav Sevcik <jarin@google.com>.

llvm-svn: 374342