[x86] make 8-bit shl undesirable

I was looking at a potential DAGCombiner fix for 1 of the regressions in D60278, and it caused severe regression test pain because x86 TLI lies about the desirability of 8-bit shift ops.

We've hinted at making all 8-bit ops undesirable for the reason in the code comment:

// TODO: Almost no 8-bit ops are desirable because they have no actual
//       size/speed advantages vs. 32-bit ops, but they do have a major
//       potential disadvantage by causing partial register stalls.

...but that leads to massive diffs and exposes all kinds of optimization holes itself.

Differential Revision: https://reviews.llvm.org/D60286

llvm-svn: 357912