Reapply "[APFloat] Fix getExactInverse for DoubleAPFloat"

The previous implementation of getExactInverse used the following check
to identify powers of two:

  // Check that the number is a power of two by making sure that only the
  // integer bit is set in the significand.
  if (significandLSB() != semantics->precision - 1)
    return false;

This condition verifies that the only set bit in the significand is the
integer bit, which is correct for normal numbers. However, this logic is
not correct for subnormal values.

APFloat represents subnormal numbers by shifting the significand right
while holding the exponent at its minimum value. For a power of two in
the subnormal range, its single set bit will therefore be at a position
lower than precision - 1. The original check would consequently fail,
causing the function to determine that these numbers do not have an
exact multiplicative inverse.

The new logic calculated this correctly but it seems that
test/CodeGen/Thumb2/mve-vcvt-fixed-to-float.ll expected the old
behavior.

Seeing as how getExactInverse does not have tests or documentation, we
conservatively maintain (and document) this behavior.

This reverts commit 47e62e846beb267aad50eb9195dfd855e160483e.