[Hexagon] Remove redundant custom selection code

[Hexagon] Remove redundant custom selection code

The clr/set/toggle-bit instructions (with the bit index given as an
immediate operand) had both, custom selection code that generated them,
and selection patterns at the same time. The selection patterns were
not used, because the custom selection code was executed first.
This patch removes the custom code in favor of the selection patterns.
The custom code handled 64-bit registers as well with an immediate bit
index, and so new patterns were added to implement that.

It was also the same case for the instruction "Rd += asr(Rs, Rt)",
except that the custom code did not offer any additional functionality,
and was simply removed.

llvm-svn: 286080