ARM-NEON: make type modifiers orthogonal and allow multiple modifiers.

The modifier system used to mutate types on NEON intrinsic definitions had a
separate letter for all kinds of transformations that might be needed, and we
were quite quickly running out of letters to use. This patch converts to a much
smaller set of orthogonal modifiers that can be applied together to achieve the
desired effect.

When merging with downstream it is likely to cause a conflict with any local
modifications to the .td files. There is a new script in
utils/convert_arm_neon.py that was used to convert all .td definitions and I
would suggest running it on the last downstream version of those files before
this commit rather than resolving conflicts manually.