[ValueTracking] peek through shuffles in ComputeNumSignBits (PR37549)

The motivating case is from PR37549:
https://bugs.llvm.org/show_bug.cgi?id=37549

The analysis improvement allows us to form a vector 'select' out of 
bitwise logic (the use of ComputeNumSignBits was added at rL345149).

The smaller test shows another InstCombine improvement - we use 
ComputeNumSignBits to add 'nsw' to shift-left. But the negative
test shows an example where we must not add 'nsw' - when the shuffle
mask contains undef elements.

Differential Revision: https://reviews.llvm.org/D53659

llvm-svn: 345429