[RISCV] Add a pattern for (sext_inreg (mul (and X, 0xffffffff), (and Y, 0xffffffff)), i32) to suppress MULW formation

We have a special pattern for
(mul (and X, 0xffffffff), (and Y, 0xffffffff)), to optimize the
ANDs to shift. But if a sext_inreg coms first, we'll form a MULW
and limit the effectiveness of the special match. So this patch
adds a larger pattern to suppress the MULW formation by emitting
a sext.w and then the same output we use for the
(mul (and X, 0xffffffff), (and Y, 0xffffffff)). This should all
get CSEd.

This is the issue I was trying to fix with D99029, but that affected
many more tests.

lib/Target/RISCV/RISCVInstrInfoM.td

@@ -104,4 +104,11 @@ let Predicates = [HasStdExtM, IsRV64, NotHasStdExtZba] in {
 // still be better off shifting both left by 32.
 def : Pat<(i64 (mul (and GPR:$rs1, 0xffffffff), (and GPR:$rs2, 0xffffffff))),
           (MULHU (SLLI GPR:$rs1, 32), (SLLI GPR:$rs2, 32))>;
+// Prevent matching the first part of this pattern to mulw. The mul here has
+// additionals users or the ANDs would have been removed. The above pattern
+// will be used for the other users. If we form a mulw we'll keep the ANDs alive
+// and they'll still become SLLI+SRLI.
+def : Pat<(sext_inreg (mul (and GPR:$rs1, 0xffffffff),
+                           (and GPR:$rs2, 0xffffffff)), i32),
+          (ADDIW (MULHU (SLLI GPR:$rs1, 32), (SLLI GPR:$rs2, 32)), 0)>;
 } // Predicates = [HasStdExtM, IsRV64, NotHasStdExtZba]

test/CodeGen/RISCV/xaluo.ll

@@ -1045,13 +1045,11 @@ define signext i32 @umulo3.i32(i32 signext %0, i32 signext %1, i32* %2) {
 ; RV64-LABEL: umulo3.i32:
 ; RV64:       # %bb.0:
 ; RV64-NEXT:    slli a1, a1, 32
-; RV64-NEXT:    srli a3, a1, 32
 ; RV64-NEXT:    slli a0, a0, 32
-; RV64-NEXT:    srli a4, a0, 32
 ; RV64-NEXT:    mulhu a0, a0, a1
-; RV64-NEXT:    srli a0, a0, 32
-; RV64-NEXT:    snez a1, a0
-; RV64-NEXT:    mulw a0, a4, a3
+; RV64-NEXT:    srli a1, a0, 32
+; RV64-NEXT:    snez a1, a1
+; RV64-NEXT:    sext.w a0, a0
 ; RV64-NEXT:    sw a1, 0(a2)
 ; RV64-NEXT:    ret
 ;