set div/rem default values to 'expensive' in TargetTransformInfo's cost model

...because that's what the cost model was intended to do.

As discussed in D12882, this fix has a temporary unintended consequence for
SimplifyCFG: it causes us to not speculate an fdiv. However, two wrongs make
PR24818 right, and two wrongs make PR24343 act right even though it's really
still wrong.

I intend to correct SimplifyCFG and add to CodeGenPrepare to account for this
cost model change and preserve the righteousness for the bug report cases.

https://llvm.org/bugs/show_bug.cgi?id=24818
https://llvm.org/bugs/show_bug.cgi?id=24343

Differential Revision: http://reviews.llvm.org/D12882

llvm-svn: 248439