fix: __builtin_bswap{16,32,64}() constantness

fix: __builtin_bswap{16,32,64}() constantness

When expanding those builtins, we directly excludes a non-constant
arg (via the expression expansion cost) and the to retrieve the
value we use const_expression_value().

But const_expression_value() is to be used for expressions that
qualify as 'integer constant expression' as per the C standard.
However we want to be able to use this builtin with all integer
expressions having a known value at compile time, even those
not qualified as true 'integer constant expression'.

Fix this by using get_expression_value_silent() instead of
const_expression_value().

Note: this bug was found by testing sparse on the git tree
      on ARM64.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Acked-by: Christopher Li <sparse@chrisli.org>