[AMDGPU] Truncate packed inline constant

If a packed inline constant is sign extended it must be truncated
after the shift. I.e. a constant (0xH0000, 0xHBC00), will be represented
as 0xFFFFFFFFBC000000 in the IR because the immediate is sign extended
to 64 bit. After the value shifted right by 16 to use it in a low part
with op_sel_hi it becomes 0xFFFFFFFFBC00 and does not qualify as inline
constant any longer.

Fixed the error and added verification code. Without the fix and with
the verification bug is causing pk_max_f16_literal.ll to fail.

Differential Revision: https://reviews.llvm.org/D45987

llvm-svn: 330752