[GISel] Allow getConstantVRegVal() to return G_FCONSTANT values.

In GISel we have both G_CONSTANT and G_FCONSTANT, but because
in GISel we don't really have a concept of Float vs Int value
the only difference between the two is where the data originates
from.

What both G_CONSTANT and G_FCONSTANT return is just a bag of bits
with the constant representation in it.

By making getConstantVRegVal() return G_FCONSTANTs bit representation
as well we allow ConstantFold and other things to operate with
G_FCONSTANT.

Adding tests that show ConstantFolding to work on mixed G_CONSTANT
and G_FCONSTANT sources.

Differential Revision: https://reviews.llvm.org/D68739

llvm-svn: 374458