PR target/99314: Fix integer signedness issue for cpymem pattern expansion.

PR target/99314: Fix integer signedness issue for cpymem pattern expansion.

Third operand of cpymem pattern is unsigned HOST_WIDE_INT, however we
are interpret that as signed HOST_WIDE_INT, that not a problem in
most case, but when the value is large than signed HOST_WIDE_INT, it
might screw up since we have using that value to calculate the buffer
size.

2021-03-05  Sinan Lin  <sinan@isrc.iscas.ac.cn>
	    Kito Cheng  <kito.cheng@sifive.com>

gcc/ChangeLog:

	* config/riscv/riscv.c (riscv_block_move_straight): Change type
	to unsigned HOST_WIDE_INT for parameter and local variable with
	HOST_WIDE_INT type.
	(riscv_adjust_block_mem): Ditto.
	(riscv_block_move_loop): Ditto.
	(riscv_expand_block_move): Ditto.