Fix #5132. Disable use of AVX2 and BMI2 instructions in LLC.

In some basic testing, it looks sufficient to disable AVX2 *or* BMI2 (not
required to disable both). Since there are known bugs with these
instructions in LLVM3.3, though, disabling both to be safe.
Merge pull request #5194 from blakejohnson/disable-static-avx2-bmi2

Fix #5132. Disable use of AVX2 and BMI2 instructions in LLC.