tree-pretty-print: Handle boolean types

tree-pretty-print: Handle boolean types

AVX512-style masks and SVE-style predicates can be difficult
to debug in gimple dumps, since the types are printed like this:

  vector(4) <unnamed type> foo;

Some important details are hidden by that <unnamed type>,
such as the number of bits in an element and whether the type
is signed or unsigned.

This patch uses an ad-hoc syntax for printing unnamed
boolean types.  Normal frontend ones should be handled
by the earlier TYPE_NAME code.

2020-05-11  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-pretty-print.c (dump_generic_node): Handle BOOLEAN_TYPEs.