Hexagon (target/hexagon) Don't overlap dest writes with source reads

Hexagon (target/hexagon) Don't overlap dest writes with source reads

When generating TCG, make sure we have read all the operand registers
before writing to the destination registers.

This is a prerequesite for short-circuiting where the source and dest
operands could be the same.

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230427230012.3800327-10-tsimpson@quicinc.com>