[Utils][x86] add an option to reduce scrubbing of shuffles with memops

[Utils][x86] add an option to reduce scrubbing of shuffles with memops

I was drafting a patch that would increase broadcast load usage,
but our shuffle scrubbing makes it impossible to see if the memory
operand offset was getting created correctly. I'm proposing to make
that an option (defaulted to 'off' for now to reduce regression
test churn).

The updated files provide examples of tests where we can now verify
that the pointer offset for a loaded memory operand is correct. We
still have stack and constant scrubbing that can obscure the operand
even if we don't scrub the entire instruction.

Differential Revision: https://reviews.llvm.org/D74775