[SelectionDAG][X86] Support f16 in getReciprocalOpName.

If the "reciprocal-estimates" attribute is present and it doesn't
contain "all", "none", or "default", we previously crashed on f16
operations.

This patch addes an 'h' suffix' to prevent the crash.

I've added simple tests that just enable the estimate for all
vec-sqrt and one test case that explicitly tests the new 'h' suffix
to override the default steps.

There may be some frontend change needed to, but I haven't checked
that yet.

Differential Revision: https://reviews.llvm.org/D120158