i965/nir/opt_peephole_ffma: Bypass fusion if any operand of fadd and fmul is a const

i965/nir/opt_peephole_ffma: Bypass fusion if any operand of fadd and fmul is a const

When both fadd and fmul instructions have at least one operand that is a
constant and it is only used once, the total number of instructions can
be reduced from 3 (1 ffma + 2 load_const) to 2 (1 fmul + 1 fadd); because
the constants will be progagated as immediate operands of fmul and fadd.

This patch detects these situations and prevents fusing fmul+fadd into ffma.

Shader-db results on i965 Haswell:

total instructions in shared programs: 6235835 -> 6225895 (-0.16%)
instructions in affected programs:     1124094 -> 1114154 (-0.88%)
total loops in shared programs:        1979 -> 1979 (0.00%)
helped:                                7612
HURT:                                  843
GAINED:                                4
LOST:                                  0

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>