The implementation of define_expand and define_insn patterns to handle
shifts in the MSP430 backend is inconsistent, resulting in missed
opportunities to make best use of the architecture's features.

There's now a single define_expand used as the entry point for all valid
shifts, and the decision to either use a helper function to perform the
shift (often required for the 430 ISA), or fall through to the
define_insn patterns can be made from that expander function.

Shifts by a constant amount have been grouped into one define_insn for
each type of shift, instead of having different define_insn patterns for
shifts by different amounts.

A new target option "-mmax-inline-shift=" has been added to allow tuning
of the number of shift instructions to emit inline, instead of using
a library helper function.

Successfully regtested on trunk for msp430-elf, ok to apply?