[flang] Fix MASKR/MASKL lowering for INTEGER(16) (#87496)

The all one masks was not properly created for i128 types because
builder.createIntegerConstant ended-up truncating -1 to something
positive.

Add a builder.createAllOnesInteger/createMinusOneInteger helpers and use
them where createIntegerConstant(..., -1) was used.
Add an assert in createIntegerConstant to catch negative numbers for
i128 type.