middle-end: Improve RTL expansion in expand_mul_overflow,

middle-end: Improve RTL expansion in expand_mul_overflow,

This patch improves the RTL that the middle-end generates for testing
signed overflow following a widening multiplication.  During this
expansion the middle-end generates a truncation which can get used
multiple times.  Placing this intermediate value in a pseudo register
reduces the amount of code generated on platforms where this truncation
requires an explicit instruction.

2020-07-11  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog:
	* internal-fn.c (expand_mul_overflow): When checking for signed
	overflow from a widening multiplication, we access the truncated
	lowpart RES twice, so keep this value in a pseudo register.