[Inliner] Restrict soft-float inlining penalty.

The penalty is currently getting applied in a bunch of places where it
doesn't make sense, like bitcasts (which are free) and calls (which
were getting the call penalty applied twice). Instead, just apply the
penalty to binary operators and floating-point casts.

While I'm here, also fix getFPOpCost() to do the right thing in more
cases, so we don't have to dig into function attributes.

Differential Revision: https://reviews.llvm.org/D41522

llvm-svn: 321332