PR18617 - Incorrect expression bytecode generated for narrowing conversions

PR18617 - Incorrect expression bytecode generated for narrowing conversions

The existing code preserves 'from' bits, which is incorrect.  E.g.

 (gdb) maint agent-eval (char)255L
 Scope: 0x4008d6
 Reg mask: 00
   0  const16 255
   3  ext 64
   5  end

'ext 64' should be 'ext 8'; this bytecode evaluates to 255 instead of
the correct result of -1.  The fix is simple.  I ran the entire test
suite on x86-64 and there were no new test failures.

gdb/ChangeLog:
2015-07-08  Robert O'Callahan  <robert@ocallahan.org>

	PR exp/18617
	* ax-gdb.c (gen_conversion): Extend to 'to' bits, not 'from'.

gdb/testsuite/ChangeLog:
2015-07-08  Robert O'Callahan  <robert@ocallahan.org>

	PR exp/18617
	* gdb.trace/ax.exp: Add test.