Optimize 'for-range-by' loop when 'by' is literal number

[zdenko]

Fixes #5015. Simplify and optimize for-range-by loop when step is a literal number.

for x in [1..a] by 1
# for (x = i = 1, ref = a; i <= ref; x = i += 1)

for x in [a..1] by 1
# for (x = i = ref = a; i <= 1; x = i += 1)

for x in [a..b] by 1
# for (x = i = ref = a, ref1 = b; i <= ref1; x = i += 1)

for x in [1..2] by 1
# for (x = i = 1; i <= 2; x = i += 1)

for x in [1..a] by -1
# for (x = i = 1, ref = a; i >= ref; x = i += -1)

for x in [a..1] by -1
# for (x = i = ref = a; i >= 1; x = i += -1)

for x in [a..b] by -1
# for (x = i = ref = a, ref1 = b; i >= ref1; x = i += -1)

for x in [2..1] by -1
# for (x = i = 2; i >= 1; x = i += -1)

[GeoffreyBooth]

isnt