When lowering vector shifts a check is performed to see if the value to shift by

When lowering vector shifts a check is performed to see if the value to shift by
is an immediate, in this check the value is negated and stored in and int64_t.
The value can be -2^63 yet the result cannot be stored in an int64_t and this
gives some undefined behaviour causing failures. The negation is only necessary
when the values is within a certain range and so it should not need to negate
-2^63, this patch introduces this and also a regression test.

Differential Revision: http://reviews.llvm.org/D11408

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243100 91177308-0d34-0410-b5e6-96231b3b80d8