AMDGPU/GISel: Factor out AMDGPURegisterBankInfo::buildReadFirstLane

A later change will add a 3rd user, so factoring out the common code
seems useful.

Reorganizing the executeInWaterfallLoop causes some more COPYs to be
generated, but those all fold away during instruction selection.
Generating the comparisons uses generic instructions over machine
instructions now which admittedly shouldn't make a difference
(though it should make it easier to move the waterfall loop generation
to another place).

(Resubmit with missing test added.)

Differential Revision: https://reviews.llvm.org/D125324