Rename the operation of turning a float type into an

Rename the operation of turning a float type into an
integer of the same type.  Before it was "promotion",
but this is confusing because it is quite different
to promotion of integers.  Call it "softening" instead,
inspired by "soft float".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52546 91177308-0d34-0410-b5e6-96231b3b80d8