[AMDGPU] Don't set src mods on permlane16

v_permlane16_b32 and v_permlanex16_b32 should not set abs and neg src
modifiers on any input, but they can set op_sel on src0 or src1 to
represent fi or bc when desired. The ISel patterns were setting
the src_modifier bits to -1, effectively setting abs and neg as well,
whenever it was intended to set op_sel, due to an error in ISel. ISel
should now correctly only set the op_sel bits.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D144519