Add DW_OP_LLVM_select_bit_piece DWARF operation

Add DW_OP_LLVM_select_bit_piece DWARF operation

Second more complex DWARF expression operation, that is usefull for
SIMD/SIMT like architectures is DW_OP_LLVM_select_bit_piece. This
operation pops three stack entries, where the first must be an integral
type value that represents a bit mask, the second must be a location
description that represents the one-location description and the third
must be a location description that represents the zero-location
description.

Resulting composite location description contains a given number of
pieces of a given bit size, created with parts from either of the two
location description, based on the bit mask.

gdb/ChangeLog:

        * compile/compile-loc2c.c (compute_stack_depth_worker): Add
        new DW_OP_LLVM_select_bit_piece operation support.
        * dwarf2/expr.c (dwarf_expr_context::create_select_composite):
        New method that creates the select bit piece composite.
        (dwarf_location::slice): New method.
        (dwarf_composite::slice): New method.
        (dwarf_expr_context::execute_stack_op): Add new
        DW_OP_LLVM_select_bit_piece operation support.
        * dwarf2/loc.c (dwarf2_get_symbol_read_needs): Add new
        DW_OP_LLVM_select_bit_piece operation support.
        (disassemble_dwarf_expression): Add new
        DW_OP_LLVM_select_bit_piece operation support.

include/ChangeLog:

        * dwarf2.def: Add new DW_OP_LLVM_select_bit_piece enumeration.

gdb/testsuite/ChangeLog:

        * gdb.dwarf2/dw2-llvm-select-bit-piece.exp: New test.
        * lib/dwarf.exp: Add new DW_OP_LLVM_select_bit_piece operation
        support.