[SPIR-V] Fix 64-bit integer literal printing (#66686)

Previously, the SPIR-V instruction printer was always printing the first
operand of an `OpConstant`'s literal value as one of the fixed operands.
This is incorrect for 64-bit values, where the first operand is actually
the value's lower-order word and should be combined with the following
higher-order word before printing.

This change fixes that issue by waiting to print the last fixed operand
of `OpConstant` instructions until the variadic operands are ready to be
printed, then using `NumFixedOps - 1` as the starting operand index for
the literal value operands.

Depends on D156049