progs/..., symm/...: Fix 32-bit right-shift idiom.

This one has a long and troubled history.  Writing

	x >> 32

is undefined behaviour if x is only 32 bits wide.  On the other hand, if
it's /not/, then this is necessary to get hold of the upper bits.

The obvious escape plan is to write

	(x >> 16) >> 16

(the parentheses are unfortunately necessary), but some Microsoft
compilers managed do bungle compiling this: they merged the two shifts
together and then decided that a shift by 32 places was a no-op.

So I wrote

	((x&~MASK32) >> 16) >> 16

which stood for many years.  Unfortunately this is really wrong too: if
x is wider than 32 bits, that's nice, but MASK32 /isn't/ necessarily, so
~MASK32 is all-bits zero and the high bits of x are just lost.

Fix this by casting MASK32 to the-type-of-x before inverting it.

Ugh.