[MLIR] prefer /bin/sh over /bin/bash for simple test scripts

[dtzWill]

Summary

These scripts do not appear to require bash, and while /bin/sh
is not guaranteed either it's more commonly available.

Fixes tests on NixOS and in certain sandbox build environments.

mlir/test/mlir-reduce/failure-test.sh

-#!/bin/bash
+#!/bin/sh
 # Tests for the keyword "failure" in the stderr of the optimization pass
 mlir-opt $1 -test-mlir-reducer > /tmp/stdout.$$ 2>/tmp/stderr.$$
 
 if [ $? -ne 0 ] && grep 'failure' /tmp/stderr.$$; then
   exit 1
   #Interesting behavior
 else 
   exit 0
 fi

mlir/test/mlir-reduce/test.sh

-#!/bin/bash
+#!/bin/sh
 
 #Replicate bug
 
 #Interesting behavior
-exit 1
+exit 1