[x86/SLH] Remove complex SHRX-based post-load hardening.

This code was really nasty, had several bugs in it originally, and
wasn't carrying its weight. While on Zen we have all 4 ports available
for SHRX, on all of the Intel parts with Agner's tables, SHRX can only
execute on 2 ports, giving it 1/2 the throughput of OR.

Worse, all too often this pattern required two SHRX instructions in
a chain, hurting the critical path by a lot.

Even if we end up needing to safe/restore EFLAGS, that is no longer so
bad. We pay for a uop to save the flag, but we very likely get fusion
when it is used by forming a test/jCC pair or something similar. In
practice, I don't expect the SHRX to be a significant savings here, so
I'd like to avoid the complex code required. We can always resurrect
this if/when someone has a specific performance issue addressed by it.

llvm-svn: 337781

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