Daishi Nakajima authored Jan 26, 2017 and Ben Gamari committed Jan 26, 2017

In `x shiftR y`, any of the following conditions cause an abort:
- `x` is a negative big integer
- The size of `x` and `y` is a multiple of `GMP_NUMB_BITS`
- The bit of the absolute value of `x` is filled with `1`

For example:
Assuming `GMP_NUMB_BITS = 2`,  the processing of `-15 shiftR 2` is as 
follows:

1. -15 = -1111 (twos complement: 10001)
2. right shift 2 (as a positive number) -> 0011
3. Due to the shift larger than GMP_NUMB_BITS, the size of the 
destination is decreasing (2bit) -> 11
4. Add 1, and get carry: (1) 00
5. abort

I fixed it that the destination size does not decrease in such a case.

Test Plan: I tested the specific case being reported.

Reviewers: goldfire, austin, hvr, bgamari, rwbarton

Reviewed By: bgamari, rwbarton

Subscribers: mpickering, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D2998

GHC Trac Issues: #12136