Add APFloat::getExactInverse.

The idea is, that if an ieee 754 float is divided by a power of two, we can
turn the division into a cheaper multiplication. This function sees if we can
get an exact multiplicative inverse for a divisor and returns it if possible.

This is the hard part of PR9587.

I tested many inputs against llvm-gcc's frotend implementation of this
optimization and didn't find any difference. However, floating point is the
land of weird edge cases, so any review would be appreciated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128545 91177308-0d34-0410-b5e6-96231b3b80d8

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