Allow to truncate left shift with non-constant shift amount

That is pretty common for clang to produce code like
(shl %x, (and %amt, 31)). In this situation we can still perform
trunc (shl) into shl (trunc) conversion given the known value
range of shift amount.

Differential Revision: https://reviews.llvm.org/D34723

llvm-svn: 306499