LoongArch: Fix cost model for alsl

Our cost model for alsl was wrong: it matches (a + b * imm) where imm is
1, 2, 3, or 4 (should be 2, 4, 8, or 16), and it does not match
(a + (b << imm)) at all.  For the test case:

    a += c << 3;
    b += c << 3;

it caused the compiler to perform a CSE and make one slli and two add,
but we just want two alsl.

Also add a "code == PLUS" check to prevent matching a - (b << imm) as we
don't have any "slsl" instruction.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_rtx_costs): Fix the
	cost for (a + b * imm) and (a + (b << imm)) which can be
	implemented with a single alsl instruction.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/alsl-cost.c: New test.

0 files changed, 0 insertions, 0 deletions