[legalize-types] Remove stale entries from SoftenedFloats.

Summary:
When replacing a SDValue, we should remove the replaced value from
SoftenedFloats (and possibly the other maps as well?).

When we revisit a Node because it needs analyzing again, we have to
remove all result values from SoftenedFloats (and possibly other maps?).

This fixes the fp128 test failures with expensive checks for X86.

I think we probably should also remove the values from the other maps
(PromotedIntegers and so on), let me know what you think.

Reviewers: baldrick, bogner, davidxl, ab, arsenm, pirama, chh, RKSimon

Reviewed By: chh

Subscribers: danalbert, wdng, srhines, hfinkel, sepavloff, llvm-commits

Differential Revision: https://reviews.llvm.org/D29265

llvm-svn: 296964