test/codegen: tighten the TrailingZeros64 test on 386

Make the TrailingZeros64 code generation check more specific for 386.
Just checking for BSFL will match both the generic 64 bit decomposition
and the custom 386 lowering.

Change-Id: I62076f1889af0ef1f29704cba01ab419cae0c6e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/656996
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: 4a6f656c

test/codegen/mathbits.go

@@ -372,7 +372,7 @@ func TrailingZeros(n uint) int {
 func TrailingZeros64(n uint64) int {
 	// amd64/v1,amd64/v2:"BSFQ","MOVL\t\\$64","CMOVQEQ"
 	// amd64/v3:"TZCNTQ"
-	// 386:"BSFL"
+	// 386:"BSFL","JNE"
 	// arm64:"RBIT","CLZ"
 	// loong64:"CTZV"
 	// s390x:"FLOGR"