Fix known zero bits for addrspacecast.

Fix known zero bits for addrspacecast.

Currently LLVM assumes that a pointer addrspacecasted to a different addr space is equivalent to trunc or zext bitwise, which is not true. For example, in amdgcn target, when a null pointer is addrspacecasted from addr space 4 to 0, its value is changed from i64 0 to i32 -1.

This patch teaches LLVM not to assume known bits of addrspacecast instruction to its operand.

Differential Revision: https://reviews.llvm.org/D26803

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287545 91177308-0d34-0410-b5e6-96231b3b80d8