[DAGCombiner] fold shift-trunc-shift to shift-mask-trunc (2nd try)

The initial attempt (rG89633320) botched the logic by reversing
the source/dest types. Added x86 tests for additional coverage.
The vector tests show a potential improvement (fold vector load
instead of broadcasting), but that's a known/existing problem.

This fold is done in IR by instcombine, and we have a special
form of it already here in DAGCombiner, but we want the more
general transform too:
https://rise4fun.com/Alive/3jZm

Name: general
Pre: (C1 + zext(C2) < 64)
%s = lshr i64 %x, C1
%t = trunc i64 %s to i16
%r = lshr i16 %t, C2
=>
%s2 = lshr i64 %x, C1 + zext(C2)
%a = and i64 %s2, zext((1 << (16 - C2)) - 1)
%r = trunc %a to i16

Name: special
Pre: C1 == 48
%s = lshr i64 %x, C1
%t = trunc i64 %s to i16
%r = lshr i16 %t, C2
=>
%s2 = lshr i64 %x, C1 + zext(C2)
%r = trunc %s2 to i16

...because D58017 exposes a regression without this fold.

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