[x86] Remove some of the --show-mc-encoding flags from avx512 tests that
need to be updated for the new vector shuffle lowering.

After talking to Adam Nemet, Tim Northover, etc., it seems that testing
MC encodings in the same suite as the basic codegen isn't the right
approach. Instead, we're going to want dedicated MC tests for the
encodings. These encodings are starting to get in my way so I wanted to
cut them out early. The total set of instructions that should have
encoding tests added is:

  vpaddd
  vsqrtss
  vsqrtsd
  vmovlhps
  vmovhlps
  valignq
  vbroadcastss

Not too many parts of these tests were even using this. =]