[mlir][spirv] Change numeric constant's bit-extension decision to be based on type

Integer constants with bit width less than a word (e.g., i8, i16)
should be bit extended based on its type to be SPIR-V spec-compliant.
Previously, the decision was based on the most significant bit of the
value which ignores the signless semantics and causes problems when
interfacing with SPIR-V tools.

Dealing with numeric literals: the SPIR-V spec says, "If a numeric
type’s bit width is less than 32-bits, the value appears in the
low-order bits of the word, and the high-order bits must be 0 for
a floating-point type or integer type with Signedness of 0, or sign
extended for an integer type with a Signedness of 1 (similarly for the
remaining bits of widths larger than 32 bits but not a multiple of 32
bits)."

Therefore, signless integers (e.g., i8, i16) and unsigned integers
should be 0-extended, and signed integers (e.g., si8, si16) should be
sign-extended.

Patch By: mshahneo
Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D151767