i965/vec4: Make opt_vector_float reset at the top of each block

i965/vec4: Make opt_vector_float reset at the top of each block

The pass isn't really control-flow aware and you can get into case where it
tries to combine instructions from different blocks.  This can actually
lead to an assertion failure when removing unneeded instructions if part of
the vector is set in one block and part in another.  This prevents
regressions in the next commit.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>