[LegalizeTypes] Remove code for softening a float type to itself.

[LegalizeTypes] Remove code for softening a float type to itself.

This was previously used to turn fp128 operations into libcalls
on X86. This is now done through op legalization after r371672.

This restores much of this code to before r254653.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371709 91177308-0d34-0410-b5e6-96231b3b80d8