R600: Do not fold single instruction with more that 3 kcache read

It fixes around 100 tfb piglit tests and 16 glean tests.

NOTE: This is a candidate for the Mesa stable branch.

Reviewed-by: Tom Stellard <thomas.stellard at amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175183 91177308-0d34-0410-b5e6-96231b3b80d8