Avoid 8 and 16bit switch conditions on x86

This adds a `TargetLoweringBase::getSwitchConditionType` callback to
give targets a chance to control the type used in
`CodeGenPrepare::optimizeSwitchInst`.

Implement callback for X86 to avoid i8 and i16 types where possible as
they often incur extra zero-extensions.

This is NFC for non-X86 targets.

Differential Revision: https://reviews.llvm.org/D124894