Handle gcc __builtin_strcmp using 128/256 bit vectors with sse4.1, avx/avx2

Handle gcc __builtin_strcmp using 128/256 bit vectors with sse4.1, avx/avx2

* amd64 front end: redo the translation into IR for PTEST, so as to
  use only IROps which we know Memcheck can do exact instrumentation
  for.  Handling for both the 128- and 256-bit cases is has been
  changed.

* ir_opt.c: add some constant folding rules to support the above.  In
  particular, for the case `ptest %reg, %reg` (the same reg twice), we
  want rflags.C to be set to a defined-1 even if %reg is completely
  undefined.  Doing that requires folding `x and not(x)` to zero when
  x has type V128 or V256.

* memcheck/tests/amd64/rh2257546_{128,256}.c: new test cases

https://bugzilla.redhat.com/show_bug.cgi?id=2257546