[GISel]: Add helpers for easy building G_FCONSTANT along with matchers

[GISel]: Add helpers for easy building G_FCONSTANT along with matchers

Added helpers to build G_FCONSTANT, along with matching ConstantFP and
unit tests for the same.

Sample usage.

auto MIB = Builder.buildFConstant(s32, 0.5); // Build IEEESingle
For Matching the above

const ConstantFP* Tmp;
mi_match(DstReg, MRI, m_GFCst(Tmp));

https://reviews.llvm.org/D44128
reviewed by: volkan

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327152 91177308-0d34-0410-b5e6-96231b3b80d8