[X86] Correctly broadcast NaN-like integers as float on AVX.

Since r288804, we try to lower build_vectors on AVX using broadcasts of
float/double.  However, when we broadcast integer values that happen to
have a NaN float bitpattern, we lose the NaN payload, thereby changing
the integer value being broadcast.

This is caused by ConstantFP::get, to which we pass the splat i32 as
a float (by bitcasting it using bitsToFloat).  ConstantFP::get takes
a double parameter, so we end up lossily converting a single-precision
NaN to double-precision.

Instead, avoid any kinds of conversions by directly building an APFloat
from the splatted APInt.

Note that this also fixes another piece of code (broadcast of
subvectors), that currently isn't susceptible to the same problem.

Also note that we could really just use APInt and ConstantInt
throughout: the constant pool type doesn't matter much.  Still, for
consistency, use the appropriate type.

llvm-svn: 304590