Revert "Partially reverted commit 1aa73cc05a81f592c376a9f7c0ff2543079b22ca to prevent bugpoint from using wrong opt program"

This reverts commit cf26c5e6b547a6a18bd22dfa8591afb4f61dc9ef.

Reverting in order to make a nicer patch.
Prefer opt from same path as bugpoint.

This fixes problems with wrong versions of opt when the tests are run from a
build directory that is not in the path. Partially reverts commit 1aa73cc05.