r300/compiler: fix swizzle lowering with a presubtract source operand

r300/compiler: fix swizzle lowering with a presubtract source operand

If a source operand has a non-native swizzle (e.g. the KIL instruction
cannot have a swizzle other than .xyzw), the lowering pass uses one or more
MOV instructions to move the operand to an intermediate temporary with
native swizzles.

This commit fixes that the presubtract information was lost during
the lowering.

NOTE: This is a candidate for both the 7.9 and 7.10 branches.