[X86] Force floating point values in constant pool decoding to print in...

[X86] Force floating point values in constant pool decoding to print in scientific notation so they can't be confused with integers.

When the floating point constants are whole numbers they have no decimal point so look like integers, but mean something very different in something like an 'and' instruction.

Ideally we would just print a decimal point and a 0, but I couldn't see how to make APFloat::toString do that.

llvm-svn: 345488