Add a fast-path for register values. Add support for constant pool entries,

Add a fast-path for register values.  Add support for constant pool entries,
allowing us to compile this:

float %test2(float* %P) {
        %Q = load float* %P
        %R = add float %Q, 10.1
        ret float %R
}

to this:

_test2:
        lfs r2, 0(r3)
        lis r3, ha16(.CPI_test2_0)
        lfs r3, lo16(.CPI_test2_0)(r3)
        fadds f1, r2, r3
        blr

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22962 91177308-0d34-0410-b5e6-96231b3b80d8